High thermal conductivity ceramic body of aluminum nitride

ABSTRACT

A process for producing an aluminum nitride ceramic body having a composition defined and encompassed by polygon P1JFA4 but not including lines JF and A4F of FIG. 4, a porosity of less than about 10% by volume, and a thermal conductivity greater than 1.00 W/cm·K at 25° C. which comprises forming a mixture comprised of aluminum nitride powder containing oxygen, yttrium oxide, and free carbon, shaping said mixture into a compact, said mixture and said compact having a composition wherein the equivalent % of yttrium and aluminum ranges between points J and A4 of FIG. 4, said compact having an equivalent % composition of Y, Al, O and N outside the composition defined and encompassed by polgon P1JFA4 of FIG. 4, heating said compact to a temperature at which its pores remain open reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, said deoxidized compact having a composition wherein the equivalent % of Al, Y, O and N is defined and encompassed by polygon P1JFA4 but not including lines JF and A4F of FIG. 4, and sintering said deoxidized compact at a temperature of at least about 1850° C. producing said ceramic body.

This application is a continuation-in-part of copending application Ser.No. 679,414 filed on Dec. 7, 1984, in the names of Irvin Charles Husebyand Carl Francis Bobik, now abandoned.

The present invention relates to the production of a liquid phasesintered polycrystalline aluminum nitride body having a thermalconductivity greater than 1.00 W/cm.K at 25° C. and preferably at least1.42 W/cm.K at 25° C. In one aspect of the present process, aluminumnitride is deoxidized by carbon to a certain extent, and then it isfurther deoxidized and/or sintered by utilizing yttrium oxide to producethe present ceramic.

A suitably pure aluminum nitride single crystal, containing 300 ppmdissolved oxygen, has been measured to have a room temperature thermalconductivity of 2.8 W/cm.K, which is almost as high as that of BeOsingle crystal, which is 3.7 W/cm.K, and much higher than that of α-Al₂O₃ single crystal, which is 0.44 W/cm.K. The thermal conductivity of analuminum nitride single crystal is a strong function of dissolved oxygenand decreases with an increase in dissolved oxygen content. For example,the thermal conductivity of aluminum nitride single crystal having 0.8wt% dissolved oxygen, is about 0.8 W/cm.K.

Aluminum nitride powder has an affinity for oxygen, especially when itssurface is not covered by an oxide. The introduction of oxygen into thealuminum nitride lattice in aluminum nitride powder results in theformation of Al vacancies via the equation: ##EQU1## Thus, the insertionof 3 oxygen atoms on 3 nitrogen sites will form one vacancy on analuminum site. The presence of oxygen atoms on nitrogen sites willprobably have a negligible influence on the thermal conductivity of AlN.However, due to the large difference in mass between an aluminum atomand a vacancy, the presence of vacancies on aluminum sites has a stronginfluence on the thermal conductivity of AlN and, for all practicalpurposes, is probably responsible for all of the decrease in the thermalconductivity of AlN.

There are usually three different sources of oxygen in nominally pureAlN powder. Source #1 is discrete particles of Al₂ O₃. Source #2 is anoxide coating, perhaps as Al₂ O₃, coating the AlN powder particles.Source #3 is oxygen in solution in the AlN lattice. The amount of oxygenpresent in the AlN lattice in AlN powder will depend on the method ofpreparing the AlN powder. Additional oxygen can be introduced into theAlN lattice by heating the AlN powder at elevated temperatures.Measurements indicate that at ˜1900° C. the AlN lattice can dissolve˜1.2 wt% oxygen. In the present invention, by oxygen content of AlNpowder, it is meant to include oxygen present as sources #1, #2 and #3.Also, in the present invention, the oxygen present with AlN powder assources #1, #2 and #3 can be removed by utilizing free carbon, and theextent of the removal of oxygen by carbon depends largely on thecomposition desired in the resulting sintered body.

According to the present invention, aluminum nitride powder can beprocessed in air and still produce a ceramic body having a thermalconductivity greater than 1.00 W/cm.K at 25° C., and preferably at least1.42 W/cm.K at 25° C.

In one embodiment of the present invention, the aluminum nitride in acompact comprised of particulate aluminum nitride of known oxygencontent, free carbon and yttrium oxide, is deoxidized by carbon toproduce a desired equivalent composition of Al, N, Y and O, and thedeoxidized compact is sintered by means of a liquid phase containingmostly Y and O and a smaller amount of Al and N.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the figures accompanying and forming apart of the specification in which:

FIG. 1 is a composition diagram (also shown as FIG. 1 in copending Ser.No. 553,213, filed on Nov. 18, 1983) showing the subsolidus phaseequilibria in the reciprocal ternary system comprised of AlN, YN, Y₂ O₃and Al₂ O₃. FIG. 1 is plotted in equivalent % and along each axis ofordinates the equivalent % of oxygen is shown (the equivalent % ofnitrogen is 100% minus the equivalent % of oxygen). Along the axis ofabscissas, the equivalent % of yttrium is shown (the equivalent % ofaluminum is 100% minus the equivalent % of yttrium). In FIG. 1, lineABCDEF but not lines CD and EF encompasses and defines the compositionof the sintered body of Ser. No. 553,213. FIG. 1 also shoWs an exampleof an ordinates-joining straight line ZZ' joining the oxygen contents ofan YN additive and an aluminum nitride powder. From the given equivalent% of yttrium and Al at any point on an ordinates-joining line passingthrough the polygon ABCDEF, the required amounts of yttrium additive andAlN for producing the composition of that point on the ordinates-joiningline can be calculated;

FIG. 2 is an enlarged view of the section of FIG. 1 showing thecomposition of the polycrystalline body of Ser. No. 553,213;

FIG. 3 is a composition diagram showing the subsolidus phase equilibriain the reciprocal ternary system comprised of AlN, YN, Y₂ O₃ and Al₂ O₃.FIG. 3 is plotted in equivalent % and along each axis of ordinates theequivalent % of oxygen is shown (the equivalent % of nitrogen is 100%minus the equivalent % of oxygen). Along the axis of abscissas, theequivalent % of yttrium is shown (the equivalent % of aluminum is 100%minus the equivalent % of yttrium). In FIG. 3, line, i.e. polygon,P1JFA4 but not including lines JF and A4F encompasses and defines thecomposition of the sintered body produced by the present process; and

FIG. 4 is an enlarged view of the section of FIG. 3 showing polygonP1JFA4.

FIGS. 1 and 3 show the same composition diagram showing the subsolidusphase equilibria in the reciprocal ternary system comprised of AlN, YN,Y₂ O₃ and Al₂ O₃ and differ only in that FIG. 1 shows the polygon ABCDEFof Ser. No. 553,213 and the line ZZ', whereas FIG. 3 shows the polygonP1JFA4. The composition defined and encompassed by the polygon ABCDEFdoes not include the composition of the present invention.

FIGS. 1 and 2 were developed algebraically on the basis of data producedby forming a particulate mixture of YN of predetermined oxygen contentand AlN powder of predetermined oxygen content, and in a few instances amixture of AlN, YN and Y₂ O₃ powders, under nitrogen gas, shaping themixture into a compact under nitrogen gas and sintering the compact fortime periods ranging from 1 to 1.5 hours at sintering temperaturesranging from about 1860° C. to about 2050° C. in nitrogen gas at ambientpressure. More specifically, the entire procedure ranging from mixing ofthe powders to sintering the compact formed therefrom was carried out ina nonoxidizing atmosphere of nitrogen.

Polygon P1JFA4 of FIGS. 3 and 4 also was developed algebraically on thebasis of data produced by the examples set forth herein as well as otherexperiments which included runs carried out in a manner similar to thatof the present examples.

The best method to plot phase equilibria that involve oxynitrides andtwo different metal atoms, where the metal atoms do not change valence,is to plot the compositions as a reciprocal ternary system as is done inFIGS. 1 and 3. In the particular system of FIGS. 1 and 3 there are twotypes of non-metal atoms (oxygen and nitrogen) and two types of metalatoms (yttrium and aluminum). The Al, Y, oxygen and nitrogen are assumedto have a valence of +3, +3, -2, and -3, respectively. All of the Al, Y,oxygen and nitrogen are assumed to be present as oxides, nitrides oroxynitrides, and to act as if they have the aforementioned valences.

The phase diagrams of FIGS. 1 to 4 are plotted in equivalent percent.The number of equivalents of each of these elements is equal to thenumber of moles of the particular element multiplied by its valence.Along the ordinate is plotted the number of oxygen equivalentsmultiplied by 100% and divided by the sum of the oxygen equivalents andthe nitrogen equivalents. Along the abscissa is plotted the number ofyttrium equivalents multiplied by 100% and divided by the sum of theyttrium equivalents and the aluminum equivalents. All compositions ofFIGS. 1 to 4 are plotted in this manner.

Compositions on the phase diagrams of FIGS. 1 to 4 can also be used todetermine the weight percent and the volume percent of the variousphases. For example, a particular point in the polygon P1JFA4 in FIG. 3or 4 can be used to determine the phase composition of thepolycrystalline body at that point.

FIGS. 1 to 4 show the composition and the phase equilibria of thepolycrystalline body in the solid state.

In copending U.S. patent application Ser. No. 553,213 entitled "HighThermal Conductivity Aluminum Nitride Ceramic Body" filed on Nov. 18,1983, in the names of Irvin Charles Huseby and Carl Francis Bobik andassigned to the assignee hereof and incorporated herein by reference,there is disclosed the process for producing a polycrystalline aluminumnitride ceramic body having a composition defined and encompassed byline ABCDEF but not including lines CD and EF of FIG. 1 therein (alsoshown as prior art FIG. 1 herein), a porosity of less than about 10% byvolume of said body and a thermal conductivity greater than 1.0 W/cm.Kat 22° C. which comprises forming a mixture comprised of aluminumnitride powder and an yttrium additive selected from the groupconsisting of yttrium, yttrium hydride, yttrium nitride and mixturesthereof, said aluminum nitride and yttrium additive having apredetermined oxygen content, said mixture having a composition whereinthe equivalent % of yttrium, aluminum, nitrogen and oxygen is definedand encompassed by line ABCDEF but not including lines CD and EF in FIG.1, shaping said mixture into a compact, and sintering said compact at atemperature ranging from about 1850° C. to about 2170° C. in anatmosphere selected from the group consisting of nitrogen, argon,hydrogen and mixtures thereof to produce said polycrystalline body.

Copending Ser. No. 553,213 also discloses a polycrystalline body havinga composition comprised of from greater than about 1.6 equivalent %yttrium to about 19.75 equivalent % yttrium, from about 80.25 equivalent% aluminum up to about 98.4 equivalent % aluminum, from greater thanabout 4.0 equivalent % oxygen to about 15.25 equivalent % oxygen andfrom about 84.75 equivalent % nitrogen up to about 96 equivalent %nitrogen.

Copending Ser. No. 553,213 also discloses a polycrystalline body havinga phase composition comprised of AlN and a second phase containing Y andO wherein the total amount of said second phase ranges from greater thanabout 4.2% by volume to about 27.3% by volume of the total volume ofsaid body, said body having a porosity of less than about 10% by volumeof said body and a thermal conductivity greater than 1.0 W/cm.K at 22°C.

Briefly stated, the present process for producing the present sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by line, i.e. polygon, P1JFA4 but not includinglines JF and A4F of FIGS. 3 or 4, a porosity of less than about 10% byvolume, and preferably less than about 4% by volume, of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C., and preferablyat least 1.42 W/cm.K at 25° C. comprises the steps:

(a) forming a mixture comprised of aluminum nitride powder containingoxygen, yttrium oxide or precursor therefor, and a carbonaceous additiveselected from the group consisting of free carbon, a carbonaceousorganic material and mixtures thereof, said carbonaceous organicmaterial thermally decomposing at a temperature ranging from about 50°C. to about 1000° C. to free carbon and gaseous product of decompositionwhich vaporizes away, shaping said mixture into a compact, said mixtureand said compact having a composition wherein the equivalent % ofyttrium and aluminum ranges between points J and A4 of FIGS. 3 or 4,which is from greater than about 0.3 equivalent % to less than about 2.5equivalent % yttrium and from greater than about 97.5 equivalent % toless than about 99.7 equivalent % aluminum, said compact having anequivalent % composition of Y, Al, O and N outside the compositiondefined and encompassed by polygon P1JFA4 of FIGS. 3 or 4,

(b) heating said compact in a nonoxidizing atmosphere at a temperatureup to about 1200° C. thereby providing yttrium oxide and free carbon,

(c) heating said compact in a nitrogen-containing nonoxidizingatmosphere at a temperature ranging from about 1350° C. to a temperaturesufficient to deoxidize the compact but below its pore closingtemperature reacting said free carbon with oxygen contained in saidaluminum nitride producing a deoxidized compact, said deoxidized compacthaving a composition wherein the equivalent % of Al, Y, O and N isdefined and encompassed by polygon P1JFA4 but not including lines JF andA4F of FIG. 3 or 4, said free carbon being in an amount which producessaid deoxidized compact, and

(d) sintering said deoxidized compact in a nitrogen-containingnonoxidizing atmosphere at a temperature of at least about 1850° C.producing said polycrystalline body.

In the present process, the composition of the deoxidized compact inequivalent % is the same as or does not differ significantly from thatof the resulting sintered body in equivalent %.

In the present invention, oxygen content can be determined by neutronactivation analysis.

By weight % or % by weight of a component herein, it is meant that thetotal weight % of all the components is 100%.

By ambient pressure herein, it is meant atmospheric or about atmosphericpressure.

By specific surface area or surface area of a powder herein, it is meantthe specific surface area according to BET surface area measurement.

Briefly stated, in one embodiment, the present process for producing asintered polycrystalline aluminum nitride ceramic body having acomposition defined and encompassed by line, i.e. polygon, A3JFA2 butnot including lines A3J, JF and A2F of FIGS. 3 or 4, a porosity of lessthan about 10% by volume, and preferably less than about 2% by volume,of said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C., and preferably greater than 1.42 W/cm.K at 25° C. comprises thesteps:

(a) forming a mixture comprised of aluminum nitride power containingoxygen, yttrium oxide or precursor therefor, and a carbonaceous additiveselected from the group consisting of free carbon, a carbonaceousorganic material and mixtures thereof, said carbonaceous organicmaterial thermally decomposing at a temperature ranging from about 50°C. to about 1000° C. to free carbon and gaseous product of decompositionwhich vaporizes away, said free carbon having a specific surface areagreater than about 100 m² /g, the aluminum nitride powder in saidmixture having a specific surface area ranging from about 3.4 m² /g toabout 6 m² /g, shaping said mixture into a compact, said mixture andsaid compact having a composition wherein the equivalent % of yttriumand aluminum ranges between points J and A2 of FIGS. 3 or 4, which isfrom greater than about 0.65 equivalent % to less than about 2.5equivalent % yttrium and from greater than about 97.5 equivalent % toless than about 99.35 equivalent % aluminum, said compact having anequivalent % composition of Y, Al, O and N outside the compositiondefined and encompassed by polygon P1JFA4 of FIGS. 3 or 4, the aluminumnitride in said compact containing oxygen in an amount ranging fromgreater than about 1.42% by weight to less than about 4.70% by weight ofsaid aluminum nitride,

(b) heating said compact in a nonoxidizing atmosphere at a temperatureup to about 1200° C. thereby providing yttrium oxide and free carbon,

(c) heating said compact at ambient pressure in a nitrogen-containingnonoxidizing atmosphere containing at least about 25% by volume nitrogenat a temperature ranging from about 1350° C. to a temperature sufficientto deoxidize the compact but below its pore closing temperature reactingsaid free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon A3JFA2 but not including lines A3J, JF and A2F ofFIG. 3 or 4, the aluminum nitride in said compact before saiddeoxidation by said carbon having an oxygen content ranging from greaterthan about 1.42% by weight to less than about 4.70% by weight of saidaluminum nitride, said free carbon being in an amount which producessaid deoxidized compact, and

(d) sintering said deoxidized compact at ambient pressure in anitrogen-containing nonoxidizing atmosphere containing at least about25% by volume nitrogen at a temperature ranging from about 1885° C. toabout 1970° C., in one embodiment from about 1885° C. to about 1950° C.,in another embodiment from about 1890° C. to about 1950° C., in anotherembodiment from about 1895° C. to about 1950° C., and yet in anotherembodiment from about 1940° C. to about 1970° C., producing saidpolycrystalline body.

Briefly stated, in another embodiment, the present process for producingthe present sintered polycrystalline aluminum nitride ceramic bodyhaving a composition defined and encompassed by line, i.e. polygon,P1JFA4 but not including lines JF and A4F of FIGS. 3 or 4, a porosity ofless than about 10% by volume, and preferably less than about 4% byvolume, of said body and a thermal conductivity greater than 1.00 W/cm.Kat 25° C. and preferably greater than 1.42 W/cm.K at 25° C. comprisesthe steps:

(a) processing an aluminum nitride powder into a compact for deoxidationby free carbon by providing an aluminum nitride powder having an oxygencontent ranging up to about 4.4% by weight of said aluminum nitridepowder, forming a mixture comprised of said aluminum nitride powder,yttrium oxide or precursor therefor, and a carbonaceous additiveselected from the group consisting of free carbon, a carbonaceousorganic material and mixtures thereof, said carbonaceous organicmaterial thermally decomposing at a temperature ranging from about 50°C. to about 1000° C. to free carbon and gaseous product of decompositionwhich vaporizes away, shaping said mixture into a compact, said mixtureand said compact having a composition wherein the equivalent % ofyttrium and aluminum ranges between points J and A4 of FIGS. 3 or 4,which is from greater than about 0.3 equivalent % to less than about 2.5equivalent % yttrium and from greater than about 97.5 equivalent % toless than about 99.7 equivalent % aluminum, said compact having anequivalent % composition of Y, Al, O and N outside the compositiondefined and encompassed by polygon P1JFA4 of FIGS. 3 or 4, during saidprocessing said aluminum nitride picking up oxygen, the oxygen contentof said aluminum nitride in said compact before said deoxidation bycarbon ranging from greater than about 1.0% by weight and usuallygreater than about 1.42% by weight, up to about 4.70% by weight of saidaluminum nitride,

(b) heating said compact in a nonoxidizing atmosphere at a temperatureup to about 1200° C. thereby providing yttrium oxide and free carbon,

(c) heating said compact in a nitrogen-containing nonoxidizingatmosphere at a temperature ranging from about 1350° C. to a temperaturesufficient to deoxidize the compact but below its pore closingtemperature reacting said free carbon with oxygen contained in saidaluminum nitride producing a deoxized compact, said deoxidized compacthaving a composition wherein the equivalent % of Al, Y, O and N isdefined and encompassed by polygon P1JFA4 but not including lines JF andA4F of FIG. 3 or 4, said free carbon being in an amount which producessaid deoxidized compact, and

(d) sintering said deoxidized compact in a nitrogen-containingnonoxidizing atmosphere at a temperature of at least about 1850° C.producing said polycrystalline body.

Briefly stated, in another embodiment, the present process for producinga sintered polycrystalline aluminum nitride ceramic body having acomposition defined and encompassed by polygon A3JFA2 but not includinglines A3J, JF and A2F of FIGS. 3 or 4, a porosity of less than about 10%by volume, and preferably less than about 2% by volume of said body anda thermal conductivity greater than 1.00 W/cm.K at 25° C. and preferablygreater than 1.42 W/cm.K at 25° C. comprises the steps:

(a) processing an aluminum nitride powder into a compact for deoxidationby free carbon by providing an aluminum nitride powder having an oxygencontent ranging from greater than about 1.00% by weight to less thanabout 4.4% by weight of said aluminum nitride powder, forming a mixturecomprised of said aluminum nitride powder, yttrium oxide or precursortherefor, and a carbonaceous additive selected from the group consistingof free carbon, a carbonaceous organic material and mixtures thereof,said carbonaceous organic material thermally decomposing at atemperature ranging from about 50° C. to about 1000° C. to free carbonand gaseous product of decomposition which vaporizes away, said freecarbon having a specific surface area greater than abpit 100m² /g thealuminum nitride powder in said mixture having a specific surface arearanging from about 3.4 m² /g to about 6 m² /g, shaping said mixture intoa compact, said mixture and said compact having a composition whereinthe equivalent % of yttrium and aluminum ranges between points J and A2of FIGS. 3 or 4, which is from greater than about 0.65 equivalent % toless than about 2.5 equivalent % yttrium and from greater than about97.5 equivalent % to less than about 99.35 equivalent % aluminum, saidcompact having an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon P1JFA4 of FIGS. 3 or 4,during said processing said aluminum nitride picking up oxygen, theoxygen content of said aluminum nitride in said compact before saiddeoxidation by carbon ranging from greater than about 1.42% by weight upto about 4.70% by weight of said aluminum nitride and being greater thansaid oxygen content of said starting aluminum nitride powder by anamount ranging from greater than about 0.03% by weight up to about 3.00%by weight of said aluminum nitride,

(b) heating said compact in a nonoxidizing atmosphere at a temperatureup to about 1200° C. thereby providing yttrium oxide and free carbon,

(c) heating said compact at ambient pressure in a nitrogen-containingnonoxidizing atmosphere containing at least about 25% by volume nitrogenat a temperature ranging from about 1350° C. to a temperature sufficientto deoxidize the compact but below its pore closing temperature therebyreacting said free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon A3JFA2 but not including lines A3J, JF and A2F ofFIG. 3 or 4, the aluminum nitride in said compact before saiddeoxidation by said carbon having an oxygen content ranging from greaterthan about 1.42% by weight to less than about 4.70% by weight of saidaluminum nitride, said free carbon being in an amount which producessaid deoxidized compact, and

(d) sintering said deoxidized compact at ambient pressure in anitrogen-containing nonoxidizing atmosphere containing at least about25% by volume nitrogen at a temperature ranging from about 1885° C. toabout 1970° C., in one embodiment from about 1885° C. to about 1950° C.,in another embodiment from about 1890° C. to about 1950° C., in anotherembodiment from about 1895° C. to about 1950° C., and yet in anotherembodiment from about 1940° C. to about 1970° C., producing saidpolycrystalline body.

In one embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon P1A3A2A4 but excluding lines P1A3,A3A2 and A2A4 of FIG. 4, the mixture and compact have a compositionwherein the equivalent % of yttrium and aluminum ranges from less thanpoint A3 up to point A4 of FIG. 4, i.e. the yttrium ranges from greaterthan about 0.3 equivalent % to less than about 0.85 equivalent % and thealuminum ranges from greater than about 99.15 equivalent % to less thanabout 99.7 equivalent %.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined by line P1A3 of FIG. 4, the mixture and compact have acomposition wherein the equivalent % of yttrium and aluminum ranges frompoint P1 to point A3, i.e. the yttrium ranges from about 0.35 equivalent% to about 0.85 equivalent % and the aluminum ranges from about 99.15equivalent % to about 99.65 equivalent %.

In another embodiment of the present process to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined by line A3J but excluding point J of FIG. 4, the mixture andcompact have a composition wherein the equivalent % of yttrium andaluminum ranges from point A3 up to point J, i.e. the yttrium rangesfrom about 0.85 equivalent % to less than about 2.5 equivalent % and thealuminum ranges from greater than about 97.5 equivalent % to about 99.15equivalent %.

More specifically, in one embodiment of the present process to produce asintered polycrystalline aluminum nitride ceramic body having acomposition defined and encompassed by polygon A3JFA2 but not includinglines A3J, JF and A2F of FIG. 4 and a porosity of less than 1% by volumeof the body, the free carbon has a specific surface area greater thanabout 100 m² /g, the aluminum nitride in said mixture has a specificsurface area ranging from about 3.5 m² /g to about 6.0 m² /g, all firingof the compact is carried out in nitrogen, and at a sinteringtemperature ranging from about 1890° C. to about 1950° C., the resultingsintered body has a thermal conductivity greater than 1.42 W/cm.K at 25°C., and at a sintering temperature ranging from about 1895° C. to about1950° C., the resulting sintered body which contains carbon in an amountof less than about 0.04% by weight of the sintered body has a thermalconductivity greater than about 1.53 W/cm.K at 25° C.

In another embodiment of the present process, to produce a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon A3JFA2 but excluding lines JF and A2Fof FIG. 4 which contains carbon in an amount of less than about 0.04% byweight of the sintered body and has a thermal conductivity greater than1.57 W/cm.K at 25° C. and a porosity of less than 1% by volume of thebody, the aluminum nitride in said mixture has a specific surface arearanging from about 3.4 m² /g to about 6.0 m² /g, the free carbon has aspecific surface area greater than 100 m² /g, all firing of the compactis carried out in nitrogen and the sintering temperature ranges fromabout 1940° C. to about 1970° C.

In yet another embodiment of the present process, said mixture and saidcompact have a composition wherein the equivalent % of yttrium andaluminum ranges from point A3 up to point J of FIG. 4, said yttrium insaid compact ranges from about 0.85 equivalent % to less than about 2.5equivalent %, said aluminum in said compact ranges from greater thanabout 97.5 equivalent % to about 99.15 equivalent %, and said sinteredbody and said deoxidized compact are comprised of a composition whereinthe equivalent percent of Al, Y, O and N is defined by line A3J butexcluding point J of FIG. 4, said free carbon has a specific surfacearea greater than about 100 m² /g, said aluminum nitride powder in saidmixture has a specific surface area ranging from about 3.5 m² /g toabout 6.0 m² /g, said firing atmosphere is nitrogen, said sinteringtemperature ranges from about 1890° C. to about 1950° C. to produce asintered body having a porosity of less than 2% by volume of thesintered body, or said sintering temperature ranges from about 1895° C.to about 1950° C. to produce a sintered body having a porosity of lessthan 1% by volume of the sintered body, and said sintered body has athermal conductivity greater than 1.43 W/cm.K at 25° C.

The calculated compositions of particular points in FIG. 3 or 4 in thepolygon P1JFA4 are shown in Table I as follows:

                  TABLE I                                                         ______________________________________                                        Composition                                                                   (Equivalent                                                                   %)             Vol % and (Wt %) of Phases*                                    Point                                                                              Y       Oxygen    AlN     Y.sub.4 Al.sub.2 O.sub.9                                                              YAlO.sub.3                             ______________________________________                                        P    0.55     1.15     98.7(98.2)                                                                            1.3(1.8)                                                                              --                                     A3   0.85    1.6       97.9(97.2)                                                                            2.1(2.8)                                                                              --                                     J    2.5     4.1       94.0(91.9)                                                                            6.0(8.1)                                                                              --                                     F    1.6     4.0       95.8(93.8)                                                                            --      4.2(6.2)                               A2   0.65    2.1       98.3(97.4)                                                                            --      1.7(2.6)                               A1   0.4     1.6       98.9(98.4)                                                                            --      1.1(1.6)                               A4   0.3     1.4       99.2(98.8)                                                                            --      0.8(1.2)                               P1   0.35     0.85     99.2(98.8)                                                                            0.8(1.2)                                                                              --                                     ______________________________________                                         *Wt % is given in parentheses,                                                Vol % is given without parentheses                                       

The polycrystalline aluminum nitride body produced by the presentprocess has a composition defined and encompassed by polygon, i.e. line,P1JFA4 but not including lines JF and A4F of FIG. 3 or 4. The sinteredpolycrystalline body of polygon P1JFA4 but not including lines JF andA4F of FIG. 3 or 4 produced by the present process has a compositioncomprised of from greater than about 0.3 equivalent % yttrium to lessthan about 2.5 equivalent % yttrium, from greater than about 97.5equivalent % aluminum to less than about 99.7 equivalent % aluminum,from about 0.85 equivalent % oxygen to less than about 4.1 equivalent %oxygen and from greater than about 95.9 equivalent % nitrogen to about99.15 equivalent % nitrogen.

Also, the polycrystalline body having a composition defined andencompassed by polygon P1JFA4 but not including lines JF and A4F of FIG.3 or 4 is comprised of an AlN phase and a second phase which ranges inamount from greater than about 0.8% by volume for a compositionadjacent, nearest or next to point A4, to less than about 6.0% by volumefor a composition adjacent, nearest or next to point J of the totalvolume of the sintered body, and such second phase can be comprised ofY₄ Al₂ O₉ or a mixture of Y₄ Al₂ O₉ and YAlO₃. When the second phase iscomprised of Y₄ Al₂ O₉, i.e. at line P1J, it ranges in amount from about0.85% by volume to less than about 6.0% by volume of the sintered body.However, when the second phase is a mixture of second phases comprisedof YAlO₃ and Y₄ Al₂ O₉, i.e. when the polycrystalline body has acomposition defined and encompassed by polygon P1JFA4 excluding linesP1J, JF and A4F, both of these second phases are always present in atleast a trace amount, i.e. at least an amount detectable by X-raydiffraction analysis, and in such mixture, the YAlO₃ phase can rangefrom a trace amount to less than about 4.2% by volume of the sinteredbody, and the Y₄ Al₂ O₉ phase can range from a trace amount to less thanabout 6.0% by volume of the total volume of the sintered body. Morespecifically, when a mixture of Y₄ Al₂ O₉ and YAlO₃ phases is present,the amount of YAlO₃ phase decreases and the amount of Y₄ Al₂ O₉ phaseincreases as the composition moves away from line A4F toward line P1J inFIG. 4. Line P1J in FIG. 4 is comprised of AlN phase and a second phasecomprised of Y₄ Al₂ O₉.

As can be seen from Table I, the polycrystalline body at point Jcomposition would have the largest amount of second phase present whichat point J would be Y₄ Al₂ O₉.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon, i.e. line, A3JFA2 but not including lines A3J,JF and A2F of FIG. 3 or 4. The sintered polycrystalline body of polygonA3JFA2 but not including lines A3J, JF and A2F of FIG. 3 or 4 producedby the present process has a composition comprised of from greater thanabout 0.65 equivalent % yttrium to less than about 2.5 equivalent %yttrium, from greater than about 97.5 equivalent % aluminum up to about99.35 equivalent % aluminum, from about 1.6 equivalent % oxygen to lessthan about 4.1 equivalent % oxygen and from greater than about 95.9equivalent % nitrogen to about 98.4 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygon A3JFA2but not including lines A3J, JF and A2F of FIG. 3 or 4 is comprised ofan AlN phase and a second phase which ranges in amount from greater thanabout 1.7% by volume to less than about 6.0% by volume of the totalvolume of the sintered body, and such second phase is comprised of amixture of Y₄ Al₂ O₉ and YAlO₃ and both of these second phases arealways present in at least a trace amount, i.e. at least an amountdetectable by X-ray diffraction analysis. Specifically, the YAlO₃ phasecan range from a trace amount to less than about 4.2% by volume of thesintered body, and the Y₄ Al₂ O₉ phase can range from a trace amount toless than about 6.0% by volume of the total volume of the sintered body.

In another embodiment, the polycrystalline aluminum nitride bodyproduced by the present process has a composition defined andencompassed by polygon, i.e. line, P1A3A2A4 but not including linesP1A3, A3A2 and A2A4 of FIG. 4 comprised of from greater than about 0.3equivalent % to less than about 0.85 equivalent % yttrium, from about99.15 equivalent % to about 99.7 equivalent % aluminum, from greaterthan about 0.85 equivalent % to less than about 2.1 equivalent % oxygenand from greater than about 97.9 equivalent % nitrogen to less thanabout 99.15 equivalent % nitrogen.

Also, the polycrystalline body defined and encompassed by polygonP1A3A2A4 but not including lines P1A3, A3A2 and A2A4 of FIG. 4 iscomprised of an AlN phase and a second phase which ranges in amount fromgreater than about 0.8% by volume to less than about 2.1% by volume ofthe total volume of the sintered body, and such second phase iscomprised of a mixture of Y₄ Al₂ O₉ and YAlO₃ and both of these secondphases are always present in at least a trade amount, i.e. at least anamount detectable by X-ray diffraction analysis. Specifically, the YAlO₃phase can range from a trace amount to less than about 1.7% by volume ofthe sintered body, and the Y₄ Al₂ O₉ phase can range from a trace amountto less than about 2.1% by volume of the total volume of the sinteredbody.

In another embodiment, the present process produces a sintered bodydefined by line P1A3 of FIG. 4 which has a phase composition comprisedof AlN and Y₄ Al₂ O₉ wherein the Y₄ Al₂ O₉ phase ranges from about 0.8%by volume to less than about 2.1% by volume of the body. Line P1A3 ofFIG. 4 has a composition comprised of from about 0.35 equivalent % toabout 0.85 equivalent % yttrium, from about 99.15 equivalent % to about99.65 equivalent % aluminum, from about 0.85 equivalent % to about 1.6equivalent % oxygen and from about 98.4 equivalent % to about 99.15equivalent % nitrogen.

In another embodiment, the present process produces a sintered bodydefined by line A3J but not including point J of FIG. 4 which has aphase composition comprised of AlN and Y₄ Al₂ O₉ wherein the Y₄ Al₂ O₉phase ranges from about 2.1% by volume to less than about 6.0% by volumeof the body. Line A3J but not including point J of FIG. 4 has acomposition comprised of from about 0. 85 equivalent % to less thanabout 2.5 equivalent % yttrium, from greater than about 97.5 equivalent% to about 99.15 equivalent % aluminum, from about 1.6 equivalent % toless than about 4.1 equivalent % oxygen and from greater than about 95.9equivalent % to about 98.4 equivalent % nitrogen.

In the present process, the aluminum nitride powder can be of commercialor technical grade. Specifically, it should not contain any impuritieswhich would have a significantly deleterious effect on the desiredproperties of the resulting sintered product. The starting aluminumnitride powder used in the present process contains oxygen generallyranging in amount up to about 4.4% by weight and usually ranging fromgreater than about 1.0% by weight to less than about 4.4% weight, i.e.up to about 4.4% by weight. Typically, commercially available aluminumnitride powder contains from about 1.5 weight % (2.6 equivalent %) toabout 3 weight % (5.2 equivalent %) of oxygen and such powders are mostpreferred on the basis of their substantially lower cost.

The oxygen content of aluminum nitride is determinable by neutronactivation analysis.

Generally, the present starting aluminum nitride powder has a specificsurface area which can range widely, and generally it ranges up to about10 m² /g. Frequently, it has a specific surface area greater than about1.0 m² /g, and more frequently of at least about 3.0 m² /g, usuallygreater than about 3.2 m² /g, and preferably at least about 3.4 m² /g.

Generally, the present aluminum nitride powder in the present mixture.i.e. after the components have been mixed, usually by milling, has aspecific surface area which can range widely, and generally it ranges toabout 10 m² /g. Frequently, it ranges from greater than about 1.0 m² /gto about 10 m² /g, and more frequently from about 3.2 m² /g to about m²/g, and preferably it ranges from about 1.5 m² /g to about 5m² /g, andin one embodiment it ranges from about 3.4 m² /g to about 5 m² /g,according to BET surface area measurement. Specifically, the minimumsintering temperature of a given composition of the present inventionincreases with increasing particle size of the aluminum nitride.

Generally, the yttrium oxide (Y₂ O₃) additive in the present mixture hasa specific surface area which can range widely. Generally, it is greaterthan about 0.4 m² /g and generally it ranges from greater than about 0.4m² /g to about 6.0 m² /g, usually from about 0.6 m² /g to about 5.0 m²/g, more usually from about 1.0 m² /g to about 5.0 m² /g, and in oneembodiment it is greater than 2.0 m² /g.

In the practice of this invention, carbon for deoxidation of aluminumnitride powder is provided in the form of free carbon which can be addedto the mixture as elemental carbon, or in the form of a carbonaceousadditive, for example, an organic compound which can thermally decomposeto provide free carbon.

The present carbonaceous additive is selected from the group consistingof free carbon, a carbonaceous organic material and mixtures thereof.The carbonaceous organic material pyrolyzes, i.e. thermally decomposes,completely at a temperature ranging from about 50° C. to about 1000° C.to free carbon and gaseous product of decomposition which vaporizesaway. In a preferred embodiment, the carbonaceous additive is freecarbon, and preferably, it is graphite.

High molecular weight aromatic compounds or materials are the preferredcarbonaceous organic materials for making the present free carbonaddition since they ordinarily give on pyrolysis the required yield ofparticulate free carbon of submicron size. Examples of such aromaticmaterials are a phenolformaldehyde condensate resin known as Novolakwhich is soluble in acetone or higher alcohols, such as butyl alcohol,as well as many of the related condensation polymers or resins such asthose of resorcinol-formaldehyde, aniline-formaldehyde, andcresol-formaldehyde. Another satisfactory group of materials arederivatives of polynuclear aromatic hydrocarbons contained in coal tar,such as dibenzanthracene and chrysene. A preferred group are polymers ofaromatic hydrocarbons such as polyphenylene or polymethylphenylene whichare soluble in aromatic hydrocarbons.

The present free carbon has a specific surface area which can rangewidely and need only be at least sufficient to carry out the presentdeoxidation. Generally, it has a specific surface area greater thanabout 10 m² /g, preferably greater than 20 m² /g, more preferablygreater than about 100 m² /g, and still more preferably greater than 150m² /g, according to BET surface area measurement to insure intimatecontact with the AlN powder for carrying out its deoxidation. Mostpreferably, the present free carbon has as high a surface area aspossible. Also, the finer the particle size of the free carbon, i.e. thehigher its surface area, the smaller are the holes or pores it leavesbehind in the deoxidized compact. Generally, the smaller the pores of agiven deoxidized compact, the lower is the amount of liquid phase whichneed be generated at sintering temperature to produce a sintered bodyhaving a porosity of less than about 1% by volume of the body.

By processing of the aluminum nitride powder into a compact fordeoxidation by free carbon, it is meant herein to include all mixing ofthe aluminum nitride powder to produce the present mixture, all shapingof the resulting mixture to produce-the compact, as well as handling andstoring of the compact before it is deoxidized by carbon. In the presentprocess, processing of the aluminum nitride powder into a compact fordeoxidation by free carbon is at least partly carried out in air, andduring such processing of the aluminum nitride powder, it picks upoxygen from air usually in an amount greater than about 0.03% by weightof the aluminum nitride, and any such pick up of oxygen is controllableand reproducible or does not differ significantly if carried out underthe same conditions. If desired, the processing of the aluminum nitridepowder into a compact for deoxidation by free carbon can be carried outin air.

In the present processing of aluminum nitride, the oxygen it picks upcan be in any form, i.e. it initially may be oxygen, or initially it maybe in some other form, such as, for example, water. The total amount ofoxygen picked up by aluminum nitride from air or other media generallyis less than about 3.00% by weight, and generally ranges from greaterthan about 0.03% by weight to less than about 3.00% by weight, andusually it ranges from about 0.10% by weight to about 1.00% by weight,and preferably it ranges from about 0.15% by weight to about 0.70% byweight, of the total weight of the aluminum nitride. Generally, thealuminum nitride in the present mixture and compact prior to deoxidationof the compact have an oxygen content of less than about 4.70% byweight, and generally ranges from greater than about 1.00% by weight,and usually greater than about 1.42% by weight to less than about 4.70%by weight, and more usually it ranges from about 2.00% by weight toabout 4.00% by weight, and frequently it ranges from about 2.20% byweight to about 3.50% by weight, of the total weight of aluminumnitride.

The oxygen content of the starting aluminum nitride powder and that ofthe aluminum nitride in the compact prior to deoxidation can bedetermined by neutron activation analysis.

In a compact, an aluminum nitride containing oxygen in an amount ofabout 4.7% by weight or more is not desirable.

In carrying out the present process, a uniform or at least asignificantly uniform mixture or dispersion of the aluminum nitridepowder, yttrium oxide powder and carbonaceous additive, generally in theform of free carbon powder, is formed and such mixture can be formed bya number of techniques. Preferably, the powders are ball milledpreferably in a liquid medium at ambient pressure and temperature toproduce a uniform or significantly uniform dispersion. The millingmedia, which usually are in the form of cylinders or balls, should haveno significant deleterious effect on the powders, and preferably, theyare comprised of steel or polycrystalline aluminum nitride, preferablymade by sintering a compact of milling media size of AlN powder and Y₂O₃ sintering additive. Generally, the milling media has a diameter of atleast about 1/4 inch and usually ranges from about 1/4 inch to about 1/2inch in diameter. The liquid medium should have no significantlydeleterious effect on the powders and preferably it is non-aqueous.Preferably, the liquid mixing or milling medium can be evaporated awaycompletely at a temperature ranging from above room or ambienttemperature to below 300° C. leaving the present mixture. Preferably,the liquid mixing medium is an organic liquid such as heptane or hexane.Also, preferably, the liquid milling medium contains a dispersant forthe aluminum nitride powder thereby producing a uniform or significantlyuniform mixture in a significantly-shorter period of milling time. Suchdispersant should be used in a dispersing amount and it should evaporateor decompose and evaporate away completely or leave no significantresidue, i.e. no residue which has a significant effect in the presentprocess, at an elevated temperature below 1000° C. Generally, the amountof such dispersant ranges from about 0.1% by weight to less than about3% by weight of the aluminum nitride powder, and generally it is anorganic liquid, preferably oleic acid.

In using steel milling media, a residue of steel or iron is left in thedried dispersion or mixture which can range from a detectable amount upto about 3.0% by weight of the mixture. This residue of steel or iron inthe mixture has no significant effect in the present process or on thethermal conductivity of the resulting sintered body.

The liquid dispersion can be dried by a number of conventionaltechniques to remove or evaporate away the liquid and produce thepresent particulate mixture. If desired, drying can be carried out inair. Drying of a milled liquid dispersion in air causes the aluminumnitride to pick up oxygen and, when carried out under the sameconditions, such oxygen pick up is reproducible or does not differsignificantly. Also, if desired, the dispersion can be spray dried.

A solid carbonaceous organic material is preferably admixed in the formof a solution to coat the aluminum nitride particles. The solventpreferably is non-aqueous. The wet mixture can then be treated to removethe solvent producing the present mixture. The solvent can be removed bya number of techniques such as by evaporation or by freeze drying, i.e.subliming off the solvent in vacuum from the frozen dispersion. In thisway, a substantially uniform coating of the organic material on thealuminum nitride powder is obtained which on pyrolysis produces asubstantially uniform distribution of free carbon.

The present mixture is shaped into a compact in air, or includesexposing the aluminum nitride in the mixture to air. Shaping of thepresent mixture into a compact can be carried out by a number oftechniques such as extrusion, injection molding, die pressing, isostaticpressing, slip casting, roll compaction or forming or tape casting toproduce the compact of desired shape. Any lubricants, binders or similarshaping aid materials used to aid shaping of the mixture should have nosignificant deteriorating effect on the compact or the present resultingsintered body. Such shaping-aid materials are preferably of the typewhich evaporate away on heating at relatively low temperatures,preferably below 400° C., leaving no significant residue. Preferably,after removal of the shaping aid materials, the compact has a porosityof less than 60% and more preferably less than 50% to promotedensification during sintering.

If the compact contains carbonaceous organic material as a source offree carbon, it is heated at a temperature ranging from about 50° C. toabout 1000° C. to pyrolyze, i.e. thermally decompose, the organicmaterial completely producing the present free carbon and gaseousproduct of decomposition which vaporizes away. Thermal decomposition ofthe carbonaceous organic material is carried out, preferably in a vacuumor at ambient pressure, in a nonoxidizing atmosphere. Preferably, thenonoxidizing atmosphere in which thermal decomposition is carried out isselected from the group consisting of nitrogen, hydrogen, a noble gassuch as argon and mixtures thereof, and more preferably it is nitrogen,or a mixture of at least about 25% by volume nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon andmixtures thereof. In one embodiment, it is a mixture of nitrogen andfrom about 1% by volume to about 5% by volume hydrogen.

The actual amount of free carbon introduced by pyrolysis of thecarbonaceous organic material can be determined by pyrolyzing theorganic material alone and determining weight loss. Preferably, thermaldecomposition of the organic material in the present compact is done inthe sintering furnace as the temperature is being raised to deoxidizingtemperature, i.e. the temperature at which the resulting free carbonreacts with the oxygen content of the AlN.

Alternately, in the present process, yttrium oxide can be provided bymeans of an yttrium oxide precursor. The term yttrium oxide precursormeans any organic or inorganic compound which decomposes completely at atemperature below about 1200° C. to form yttrium oxide and by-productgas which vaporizes away leaving no contaminants in the sintered bodywhich would be detrimental to the thermal conductivity. Representativeof the precursors of yttrium oxide useful in the present process isyttrium acetate, yttrium carbonate, yttrium oxalate, yttrium nitrate,yttrium sulfate and yttrium hydroxide.

If the compact contains a precursor for yttrium oxide, it is heated to atemperature up to about 1200° C. to thermally decompose the precursorthereby providing yttrium oxide. Such thermal decomposition is carriedout in a non-oxidizing atmosphere, preferably in a vacuum or at ambientpressure, and preferably the atmosphere is selected from the groupconsisting of nitrogen, hydrogen, a noble gas such as argon and mixturesthereof. Preferably, it is nitrogen, or a mixture of at least about 25%by volume nitrogen and a gas selected from the group consisting ofhydrogen, a noble gas such as argon and mixtures thereof. In oneembodiment, it is a mixture of nitrogen and from about 1% by volume toabout 5% by volume hydrogen.

The present deoxidation of aluminum nitride with carbon, i.e.carbon-deoxidation, comprises heating the compact comprised of aluminumnitride, free carbon and yttrium oxide at deoxidation temperature toreact the free carbon with at least a sufficient amount of the oxygencontained in the aluminum nitride to produce a deoxidized compact havinga composition defined and encompassed by polygon P1JFA4 but notincluding lines JF and A4F of FIGS. 3 or 4. This deoxidation with carbonis carried out at a temperature ranging from about 1350° C. to atemperature at which the pores of the compact remain open, i.e. atemperature which is sufficient to deoxidize the compact but below itspore closing temperature, generally up to about 1800° C., andpreferably, it is carried out at from about 1600° C. to 1650° C.

The carbon-deoxidation is carried out, preferably at ambient pressure,in a gaseous nitrogen-containing nonoxidizing atmosphere which containssufficient nitrogen to facilitate the deoxidation of the aluminumnitride. In accordance with the present invention, nitrogen is arequired component for carrying out the deoxidation of the compact.Preferably, the nitrogen-containing atmosphere is nitrogen, or it is amixture of at least about 25% by volume of nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon, andmixtures thereof. Also, preferably, the nitrogen-containing atmosphereis comprised of a mixture of nitrogen and hydrogen, especially a mixturecontaining up to about 5% by volume hydrogen.

The time required to carry out the present carbon-deoxidation of thecompact is determinable empirically and depends largely on the thicknessof the compact as well as the amount of free carbon it contains, i.e.the carbon-deoxidation time increases with increasing thickness of thecompact and with increasing amounts of free carbon contained in thecompact. Carbon-deoxidation can be carried out as the compact is beingheated to sintering temperature provided that the heating rate allowsthe deoxidation to be completed while the pores of the compact are openand such heating rate is determinable empirically. Also, to some extent,carbon deoxidation time depends on deoxidation temperature, particlesize and uniformity of the particulate mixture of the compact i.e. thehigher the deoxidation temperature, the smaller the particle size andthe more uniform the mixture, the shorter is deoxidation time.Typically, the carbon-deoxidation time ranges from about 1/4 hour toabout 1.5 hours.

Preferably, the compact is deoxidized in the sintering furnace byholding the compact at deoxidation temperature for the required time andthen raising the temperature to sintering temperature. The deoxidationof the compact must be completed before sintering closes off pores inthe compact preventing gaseous product from vaporizing away and therebypreventing production of the present sintered body.

In the present deoxidation with carbon, the free carbon reacts with theoxygen of the aluminum nitride producing carbon monoxide gas whichvaporizes away. It is believed that the following deoxidation reactionoccurs wherein the oxygen content of the aluminum nitride is given asAl₂ O₃ :

    Al.sub.2 O.sub.3 +3C+N.sub.2 →3CO.sub.(g) +2AlN     (2)

In the deoxidation effected by carbon, gaseous carbon-containing productis produced which vaporizes away thereby removing free carbon.

If the compact before deoxidation is heated at too fast a rate throughthe carbon-deoxidation temperature to sintering temperature, and suchtoo fast rate would depend largely on the composition of the compact andthe amount of carbon it contains, the present carbon-deoxidation doesnot occur, i.e. an insufficient amount of deoxidation occurs, and asignificant amount of carbon is lost by reactions (3) and/or (3A).

    C+AlN→AlCN.sub.(g)                                  (3)

    C+1/2N.sub.2 →CN.sub.(g)                            (3A)

The specific amount of free carbon required to produce the presentdeoxidized compact can be determined by a number of techniques. It canbe determined empirically. Preferably, an initial approximate amount ofcarbon is calculated from Equation (2), that is the stoichiometricamount for carbon set forth in Equation (2), and using such approximateamount, the amount of carbon required in the present process to producethe present sintered body would require one or a few runs to determineif too much or too little carbon had been added. Specifically, this canbe done by determining the porosity of the sintered body and byanalyzing it for carbon and by X-ray diffraction analysis. If thecompact contains too much carbon, the resulting deoxidized compact willbe more difficult to sinter and will not produce the present sinteredbody. If the compact contains too little carbon, X-ray diffractionanalysis of the resulting sintered body will not show any Y₄ Al₂ O₉phase and will show that its composition is not defined or encompassedby the polygon P1JFA4 not including lines JF and A4F of FIG. 4.

The amount of free carbon used to carry out the present deoxidationshould produce the present deoxidized compact leaving no significantamount of carbon in any form, i.e. no amount of carbon in any form whichwould have a significantly deleterious effect on the sintered body. Morespecifically, no amount of carbon in any form should be left in thedeoxidized compact which would prevent production of the presentsintered body, i.e. any carbon content in the sintered body should below enough so that the sintered body has a thermal conductivity greaterthan 1.00 W/°K. at 25° C. Generally, the present sintered body maycontain carbon in some form in a trace amount, i.e. generally less thanabout 0.08% by weight, preferably in an amount of less than about 0.065%by weight, more preferably less than about 0.04% by weight, and mostpreferably less than 0.03% by weight of the total weight of the sinteredbody.

A significant amount of carbon in any form remaining in the sinteredbody significantly reduces its thermal conductivity. An amount of carbonin any form greater than about 0.065% by weight of the sintered body islikely to significantly decrease its thermal conductivity.

The present deoxidized compact is densified, i.e. liquid-phase sintered,at a temperature which is a sintering temperature for the composition ofthe deoxidized compact to produce the present polycrystalline bodyhaving a porosity of less than about 10% by volume, and preferably lessthan about 4% by volume, of the sintered body. For the presentcomposition defined and encompassed by polygon PIJFA4 but excludinglines JF and A4F, this sintering temperature generally is at least about1850° C. and generally ranges from about 1850° C. to about 2050° C. withthe minimum sintering temperature increasing generally from about 1850°C. for a composition represented by a point next to point F to greaterthan about 1920° C. but less than about 1990° C. for the composition atpoint P1. Minimum sintering temperature is dependent most strongly oncomposition and less strongly on particle size.

More specifically, in the present invention, for the present deoxidizedcompact having a constant particle size, the minimum sinteringtemperature occurs at a composition represented by a point next to pointF within the polygon P1JFA4 and such temperature increases as thecomposition moves away from point F toward point P1. Specifically, forsuch a deoxidized compact having a composition defined and encompassedby polygon A3JFA2 of FIG. 4 excluding lines A3J, JF and A2F, the minimumsintering temperature is generally about 1850° C. For a deoxidizedcompact having a composition defined and encompassed by polygon P1A3A2A4excluding lines P1A3, A3A2 and A2A4, the minimum sintering temperatureincreases generally from about 1850° C. at a point adjacent, next ornearest to point A2 to generally about 1890° C. at a point adjacent,next or nearest to point P to less than about 1990° C. at point P1. Theminimum sintering temperature for a composition on line A3J of FIG. 4generally is about 1860° C. The minimum sintering temperature for acomposition on line A3P1 generally ranges from about 1860° C. at pointA3 to generally about 1900° C. at point P to less than about 1990° C. atpoint P1.

More specifically, the minimum sintering temperature is dependentlargely on the composition (i.e., position in the FIG. 4 phase diagram),the green density of the compact, i.e. the porosity of the compact afterremoval of shaping aid materials but before deoxidation, the particlesize of aluminum nitride, and to a much lesser extent the particle sizeof yttrium oxide and carbon. The minimum sintering temperature increasesas the composition moves from next or nearest to point F to point P1,and as the green density of the compact decreases, and as the particlesize of aluminum nitride and to a much lesser extent, yttrium oxide andcarbon increases. For example, for a composition represented by a pointwithin polygon P1JFA4 of FIG. 4 and nearest to point F, the minimumsintering temperature is about 1850° C. for the particle sizecombination of aluminum nitride, yttrium oxide, and carbon of about 5.0m² /g, 2.8 m² /g, and 200 m² /g, respectively.

To carry out the present liquid phase sintering, the present deoxidizedcompact contains sufficient equivalent percent of Y and O to form asufficient amount of liquid phase at sintering temperature to densifythe carbon-deoxidized compact to produce the present sintered body. Thepresent minimum densification, i.e. sintering, temperature depends onthe composition of the deoxidized compact, i.e. the amount of liquidphase it generates. Specifically, for a sintering temperature to beoperable in the present invention, it must generate at least sufficientliquid phase in the particular composition of the deoxidized compact tocarry out the present liquid phase sintering to produce the presentproduct. For a given composition, the lower the sintering temperature,the smaller is the amount of liquid phase generated, i.e. densificationbecomes more difficult with decreasing sintering temperature. However, asintering temperature higher than about 2050° C. provides no significantadvantage.

In one embodiment of the present invention, the sintering temperatureranges from about 1890° C. to about 2050° C., and in another embodimentfrom about 1880° C. to about 1950° C., and in another embodiment fromabout 1890° C. to about 1950° C., and yet in another embodiment fromabout 1885° C. about 1950° C., and still in another embodiment fromabout 1895° C. to about 1950° C., and still in another embodiment fromabout 1940° C. to about 1970° C., to produce the present polycrystallinebody.

The deoxidized compact is sintered, preferably at ambient pressure, in agaseous nitrogen-containing nonoxidizing atmosphere which contains atleast sufficient nitrogen to prevent significant weight loss of aluminumnitride. In accordance with the present invention, nitrogen is anecessary component of the sintering atmosphere to prevent anysignificant weight loss of AlN during sintering, and also to optimizethe deoxidation treatment and to remove carbon. Significant weight lossof the aluminum nitride can vary depending on its surface area to volumeratio, i.e. depending on the form of the body, for example, whether itis in the form of a thin or thick tape. As a result, generally,significant weight loss of aluminum nitride ranges from in excess ofabout 5% by weight to in excess of about 10% by weight of the aluminumnitride. Preferably, the nitrogen-containing atmosphere is nitrogen, orit is a mixture at least about 25% by volume nitrogen and a gas selectedfrom the group consisting of hydrogen, a noble gas such as argon andmixtures thereof. Also, preferably, the nitrogen-containing atmosphereis comprised of a mixture of nitrogen and hydrogen, especially a mixturecontaining from about 1% by volume to about 5% by volume hydrogen.

Sintering time is determinable empirically. Typically, sintering timeranges from about 40 minutes to about 90 minutes.

In one embodiment, i.e. the composition defined by polygon P1JFA4 butnot including lines P1J, JF and A4F of FIG. 4, where the aluminumnitride in the carbon-deoxidized compact contains oxygen, the yttriumoxide further deoxidizes the aluminum nitride by reacting with theoxygen to form Y₄ Al₂ O₉ and YAlO₃, thus decreasing the amount of oxygenin the AlN lattice to produce the present sintered body having a phasecomposition comprised of AlN and a second phase mixture comprised ofYAlO₃ and Y₄ Al₂ O₉.

In another embodiment, i.e. line P1J but excluding point J of FIG. 4,where the aluminum nitride in the carbon-deoxidized compact containsoxygen in an amount significantly smaller than that of polygon P1JFA4but not including lines P1J, JF and A4F of FIG. 4, the resultingsintered body has a phase composition comprised of AlN and Y₄ Al₂ O₉.

The present sintered polycrystalline body is a pressureless sinteredceramic body. By pressureless sintering herein it is meant thedensification or consolidation of the deoxidized compact without theapplication of mechanical pressure into a ceramic body having a porosityof less than about 10% by volume, and preferably less than about 4% byvolume.

The polycrystalline body of the present invention is liquid-phasesintered. I.e., it sinters due to the presence of a liquid phase, thatis liquid at the sintering temperature and is rich in yttrium and oxygenand contains some aluminum and nitrogen. In the present polycrystallinebody, the AlN grains have about the same dimensions in all directions,and are not elongated or disk shaped. Generally, the AlN in the presentpolycrystalline body has an average grain size ranging from about 1micron to about 20 microns. An intergranular second phase of Y₄ Al₂ O₉or a mixture of YAlO₃ and Y₄ Al₂ O₉ is present along some of the AlNgrain bounderies. The morphology of the microstructure of the presentsintered body indicates that this intergranular second phase was aliquid at the sintering temperature. As the composition approaches lineJF in FIG. 4, the amount of liquid phase increases and the AlN grains inthe present sintered body become more rounded and have a smoothersurface. As the composition moves away from line JF in FIG. 4 andapproaches line PIA4, the amount of liquid phase decreases and the AlNgrains in the present sintered body become less rounded and the cornersof the grains become sharper.

The present sintered body has a porosity of less than about 10% byvolume, and generally less than about 4% by volume of the sintered body.Preferably, the present sintered body has a porosity of less than about2% and most preferably less than about 1% by volume of the sinteredbody. Any pores in the sintered body are fine sized, and generally theyare less than about 1 micron in diameter. Porosity can be determined bystandard metallographic procedures and by standard density measurements.

The present process is a control process for producing a sintered bodyof aluminum nitride having a thermal conductivity greater than 1.00W/cm°K. at 25° C., and preferably at least or greater than 1.42 W/cm°K.at 25° C. Generally, the thermal conductivity of the presentpolycrystalline body is less than that of a high purity single crystalof aluminum nitride which is about 2.8 W/cm°K. at 25° C. If the sameprocedure and conditions are used throughout the present process, theresulting sintered body has a thermal conductivity and composition whichis reproducible or does not differ significantly. Generally, thermalconductivity increases with a decrease in volume % of second phase, adecrease in porosity and for a given composition with increase insintering temperature.

In the present process, aluminum nitride picks up oxygen in acontrollable or substantially controllable manner. Specifically, if thesame procedure and conditions are used in the present process, theamount of oxygen picked up by aluminum nitride is reproducible or doesnot differ significantly. Also, in contrast to yttrium, yttrium nitrideand yttrium hydride, yttrium oxide or the present precursor does notpick up oxygen, or does not pick up any significant amount of oxygen,from air or other media in the present process. More specifically, inthe present process, yttrium oxide does not pick up any amount of oxygenin any form from the air or other media which would have any significanteffect on the controllability or reproducibility of the present process.Any oxygen which yttrium oxide might pick up in the present process isso small as to have no effect or no significant effect on the thermalconductivity or composition of the resulting sintered body.

Examples of calculations for equivalent % are as follows:

For a starting AlN powder weighing 89.0 grams measured as having 2.3weight % oxygen, it is assumed that all of the oxygen is bound to AlN asAl₂ O₃, and that the measured 2.3 weight % of oxygen is present as 4.89weight % Al₂ O₃ so that the AlN powder is assumed to be comprised of84.65 grams AlN and 4.35 grams Al₂ O₃.

A mixture is formed comprised of 89.0 grams of the starting AlN powder,3.2 grams of Y₂ O₃ and 1.15 grams free carbon.

During processing, this AlN powder picks up additional oxygen byreactions similar to (4) and now contains 2.6 weight % oxygen.

    2 AlN+3H.sub.2 O→Al.sub.2 O.sub.3 +2NH.sub.3        (4)

The resulting compact now is comprised of the following composition:

89.11 grams AlN powder containing 2.6 weight % oxygen, (84.19g AlN+4.92g Al₂ O₃), 3.2 grams Y₂ O₃ and 1.15 grams carbon.

During deoxidation of the compact, all the carbon is assumed to reactwith Al₂ O₃ via reaction (5)

    Al.sub.2 O.sub.3 +3C+N.sub.2 →2AlN+3CO.sub.(g)      (5)

In the present invention, the carbon will not reduce Y₂ O₃, but instead,reduces Al₂ O₃.

After reaction (5) has gone to completion, the deoxidized compact now iscomprised of the following composition which was calculated on the basisof Reaction (5):

88.47 grams AlN powder containing 0.89 weight % oxygen (86.81 grams AlN+1.67 grams Al₂ O₃) and 3.2 grams Y₂ O₃.

From this weight composition, the composition in equivalent % can becalculated as follows:

    ______________________________________                                                Wt (g)      Moles      Equivalents                                    ______________________________________                                        AlN     86.81       2.118      6.354                                          Al.sub.2 O.sub.3                                                                      1.67        1.636 × 10.sup.-2                                                                  0.098                                          Y.sub.2 O.sub.3                                                                       3.20        1.417 × 10.sup.-2                                                                  0.085                                          ______________________________________                                         TOTAL EQUIVALENTS = 6.537                                                     V = Valence                                                                   ##STR1##                                                                       -                                                                            MW = molecular weight                                                         Eq = Equivalents                                                              Eq = M × V                                                              Valence:                                                                      Al + 3                                                                        V + 3                                                                         N - 3                                                                         O - 2                                                                    

    Eq % Y in deoxidized compact = (6)                                             ##STR2##                                                                      ##STR3##                                                                     Eq % O in deoxidized compact = (7)                                             ##STR4##                                                                      ##STR5##                                                                     This deoxidized compact as well as the sintered body contains about 1.30  

To produce the present sintered body containing 1.5 equivalent % Y and3.0 equivalent % O, i.e. comprised of 1.5 equivalent % Y, 98.5equivalent % Al, 3.0 equivalent % O and 97.0 equivalent % N, using anAlN powder measured as having 2.3 weight % Oxygen (4.89 weight % Al₂O₃), the following calculations for weight % from equivalent % can bemade:

100 grams=weight of AlN powder

x grams=weight of Y₂ O₃ powder

z grams=weight of Carbon powder

Assume that during processing, the AlN powder picks up additional oxygenby reaction similar to (9) and in the compact before deoxidation nowcontains 2.6 weight % oxygen (5.52 weight % Al₂ O₃) and weighs 100.12grams

2AlN+3H₂ O→Al₂ O₃ +2NH₃ (9)

After processing, the compact can be considered as having the followingcomposition:

    ______________________________________                                        Weight (g)      Moles       Equivalents                                       ______________________________________                                        AlN     94.59       2.308       6.923                                         Al.sub.2 O.sub.3                                                                       5.53       0.0542      0.325                                         Y.sub.2 O.sub.3                                                                       x           4.429 × 10.sup.-3 x                                                                   0.02657x                                    C       z            .0833z                                                   ______________________________________                                    

During deoxidation, 3 moles of carbon reduce 1 mole of Al₂ O₃ and in thepresence of N₂ form 2 moles of AlN by the reaction:

    Al.sub.2 O.sub.3 +3C+N.sub.2 →2AlN+3CO              (10)

After deoxidation, all the carbon will have reacted and the compact canbe considered as having the following composition:

    ______________________________________                                        Weight (g)     Moles         Equivalents                                      ______________________________________                                        AlN    94.59 + 2.275z                                                                            2.308  + 0.05551z                                                                           6.923 + 0.1665z                              Al.sub.2 O.sub.3                                                                      5.53 - 2.830z                                                                            0.0542 - 0.02775z                                                                           0.325 - 0.1665z                              Y.sub.2 O.sub.3                                                                      x           4.429 × 10.sup.-3 x                                                                   0.02657x                                     ______________________________________                                         ##EQU2##     Solving Equations (11) and (12) for x and z: x=4.15 grams of Y.sub.2     O.sub.3 powder

z=1.29 grams of free carbon

A body in a form or shape useful as a substrate, i.e. in the form of aflat thin piece of uniform thickness, or having no significantdifference in its thickness, usually referred to as a substrate or tape,may become non-flat, for example, warp, during sintering and theresulting sintered body may require a heat treatment after sintering toflatten it out and make it useful as a substrate. This non-flatness orwarping is likely to occur in the sintering of a body in the form of asubstrate or tape having a thickness of less than about 0.070 inch andcan be eliminated by a flattening treatment, i.e. by heating thesintered body, i.e. substrate or tape, under a sufficient appliedpressure at a temperature in the present sintering temperature range offrom about 1850° C. to about 2050° C. for a period of time determinableempirically, and allowing the sandwiched body to cool to below itssintering temperature, preferably to ambient or room temperature, beforerecovering the resulting flat substrate or tape.

Specifically, in one embodiment of this flattening process, the non-flatsubstrate or tape is sandwiched between two plates and is separated fromsuch plates by a thin layer of AlN powder, the sandwiched body is heatedto its sintering temperature, i.e. a temperature which is a sinteringtemperature for the sandwiched sintered body, preferably in the sameatmosphere used for sintering, under an applied pressure at leastsufficient to flatten the body, generally at least about 0.03 psi, for atime period sufficient to flatten the sandwiched body, and then thesandwiched body is allowed to cool to below its sintering temperaturebefore it is recovered.

One embodiment for carrying out this flattening treatment of a sinteredthin body or substrate tape comprises sandwiching the sintered non-flatsubstrate or tape between two plates of a material which has nosignificant deleterious effect thereon such as molybdenum or tungsten,or an alloy containing at least about 80% by weight of tungsten ormolybdenum. The sandwiched substrate or tape is separated from theplates by a thin layer, preferably a discontinuous coating, preferably adiscontinuous monolayer, of aluminum nitride powder preferably justsufficient to prevent the body from sticking to the surfaces of theplates during the flattening heat treatment. The flattening pressure isdeterminable empirically and depends largely on the particular sinteredbody, the particular flattening temperature and flattening time period.The flattening treatment should have no significant deleterious effecton the sintered body. A decrease in flattening temperature requires anincrease in flattening pressure or flattening time. Generally, at atemperature ranging from about 1850° C. or about 1890° C. to about 2050°C., the applied flattening pressure ranges from about 0.03 psi to about1.0 psi, preferably from about 0.06 psi to about 0.50 psi, and morepreferably from about 0.10 psi to about 0.30 psi. Typically, forexample, heating the sandwiched sintered body at the sinteringtemperature under a pressure of from about 0.03 psi to about 0.5 psi for1 hour in nitrogen produces a flat body useful as a substrate,especially as a supporting substrate for a semiconductor such as asilicon chip.

The present invention makes it possible to fabricate simple, complexand/or hollow shaped polycrystalline aluminum nitride ceramic articlesdirectly. Specifically, the present sintered body can be produced in theform of a useful shaped article without machining or without anysignificant machining, such as a hollow shaped article for use as acontainer, a crucible, a thin walled tube, a long rod, a spherical body,a tape, substrate or carrier. It is useful as a sheath for temperaturesensors. It is especially useful as a substrate for a semiconductor suchas a silicon chip. The dimensions of the present sintered body differfrom those of the unsintered body, by the extent of shrinkage, i.e.densification, which occurs during sintering.

The present ceramic body has a number of uses. In the form of a thinflat piece of uniform thickness, or having no significant difference inits thickness, i.e. in the form of a substrate or tape, it is especiallyuseful as packaging for integrated circuits and as a substrate for anintegrated circuit, particularly as a substrate for a semiconducting Sichip for use in computers.

The invention is further illustrated by the following examples whereinthe procedure was as follows, unless otherwise stated:

The starting aluminum nitride powder contained oxygen in an amount ofless than 4% by weight.

The starting aluminum nitride powder was greater than 99% pure AlNexclusive of oxygen.

In Examples 10, 11, 13, and 15 of Table III, the starting AlN powder hada surface area of 0.5 m² /g. In Example 14 of Table III the AlN powderhad a surface area of 1.6 m² /g.

In Examples 5 and 6 of Table II, and 12A and 12B of Table III, thestarting aluminum nitride powder had a surface area of 3.84 m² /g (0.479micron) and contained 2.10 wt % oxygen as determined by neutronactivation analysis.

In the remaining examples of Tables II and III, the starting aluminumnitride powder had a surface area of 4.96 m² /g (0.371 micron) andcontained 2.25 wt % oxygen as determined by neutron activation analysis.

In all of the examples of Table II and Examples 9A, 9B, 12A and 12B ofTable III, the y₂ O₃ powder, before any mixing, i.e. as received, had asurface area of about 2.75 m² /g. In Examples 10, 11 and 15 of TableIII, the Y₂ O₃ powder, before mixing, had a surface area of 0.6 m² /g.In Examples 13 and 14 of Table III, Y₂ (CO₃)₃° 3H₂ O was added as aprecursor for Y₂ O₃.

The carbon used in all of the examples of Table II was graphite and inExamples 10, 11 and 15 of Table III, it had a specific surface area of25 m² /g as listed by the vender, and in the remaining examples ofTables II and III, it had a specific surface area of 200 m² /g (0.017micron) as listed by the vendor.

Non-aqueous heptane was used to carry out the mixing, i.e. milling, ofthe powders in all of the examples of Tables II and III.

In all of the examples of Tables II and III, the milling media was hotpressed aluminum nitride in the approximate form of cubes or rectangleshaving a density of about 100%.

In Examples 1A, 1B, 2, 5 and 6 of Table II and 10, 11, 12A, 12B, 13, 14and 15 of Table III, the AlN, Y₂ O₃ and carbon powders were immersed innon-aqueous heptane containing oleic acid in an amount of about 0.7% byweight of the aluminum nitride powder in a plastic jar and vibratorymilled in the closed jar at room temperature for about 18 hours inExamples 1A, 1B and 2, and for about 16 hours in Examples 5, 6, 10, 11,12A, 12B, 13, 14 and 15, producing the given powder mixture. In theremaining examples of Tables II and III, no oleic acid was used, and theAlN, Y₂ O₃ and carbon powders were immersed in non-aqueous heptane in aplastic jar and vibratory milled in the closed jar at room temperaturefor a period of time which for Example 3 was about 91 hours, forExamples 4A and 4B, it was about 20 hours, for Examples 7A, 7B and 8, itwas about 68 hours, and for Examples 9A and 9B it was about 46 hours.

In all of the Examples of Tables II and III, the milled liquiddispersion of the given powder mixture was dried in air at ambientpressure under a heat lamp for about 20 minutes and during such drying,the mixture picked up oxygen from the air.

ln all of the Examples of Tables II and III, the dried milled powdermixture was die pressed at 5 Kpsi in air at room temperature to producea compact having a density of roughly 55% of its theoretical density.

In those examples of Tables II and III, wherein the sintered body isgiven as being of A size or of B size, the compacts were in the form ofa disk, in those examples wherein the sintered body is given as being ofC size, the compacts were in the form of a bar, and in those exampleswherein the sintered body is given as being of D size, the compacts werein the form of a substrate which was a thin flat piece, like a tape, ofuniform thickness, or of a thickness which did not differ significantly.

In Table II the composition of the mixture of powders is shown as PowderMixture whereas in Table III it is shown as Powders Added.

In all of the examples of Table II and Table III except Examples 4A, 4B,5, 6, 7A, 7B, 8, 12A, 12B, 13, 14 and 15, the given powder mixture aswell as the compact formed therefrom had a composition wherein theequivalent % of yttrium and aluminum ranged between points J and A4 ofFIG. 4.

In Examples 4A, 4B, 5, 6, 7A, 7B and 8 of Table II, and 12A, 12B, 13, 14and 15 of Table III, the given powder mixture as well as the compactformed therefrom had a composition wherein the equivalent % of yttriumand aluminum were outside the range of from point J to point A4 of FIG.4.

The equivalent % composition of Y, Al, O and N of the compacts of all ofthe Examples of Tables II and III, i.e. before deoxidation, was outsidethe composition defined and encompassed by polygon P1JFA4 of FIG. 4.

In all of the examples of Tables II and III, the aluminum nitride in thecompact before deoxidation contained oxygen in an amount ranging fromgreater than about 1.42% by weight to less than about 4.70% by weight ofthe aluminum nitride.

The composition of the deoxidized compacts of all of the Examples ofTables II and III, except Examples 4A, 4B, 5, 6, 7A, 7B, 8, 12A, 12B,13, 14 and 15, were defined and encompassed by polygon P1JFA4 of FIG. 4but did not include lines JF and A4F.

In each of the examples of Tables II and III, one compact was formedfrom the given powder mixture and was given the heat treatment shown inTables II and III. Also, the examples in Tables II and III having thesame number but including the letters A or B indicate that they werecarried out in an identical manner, i.e. the powder mixtures wereprepared and formed into two compacts in the same manner and the twocompacts were heat treated under identical conditions, i.e. the twocompacts were placed side by side in the furnace and given the same heattreatment simultaneously, and these examples numbered with an A or B maybe referred to herein by their number only.

In all of the examples of Tables II and III, the same atmosphere wasused to carry out the deoxidation of the compacts as was used to carryout the sintering of the deoxidized compact except that the atmosphereto carry out the deoxidization was fed into the furnace at a rate of 1SCFH to promote removal of the gases produced by deoxidation, and theflow rate during sintering was less than about 0.1 SCFH.

The atmosphere during all of the heat treatment in all of the examplesin Tables II and III was at ambient pressure which was atmospheric orabout atmospheric pressure.

The furnace was a molybdenum heat element furnace.

The compacts were heated in the furnace to the given deoxidationtemperature at the rate of about 100° C. per minute and then to thegiven sintering temperature at the rate of about 50° C. per minute.

The sintering atmosphere was at ambient pressure, i.e. atmospheric orabout atmospheric pressure.

At the completion of heat treatment, the samples were furnace-cooled toabout room temperature.

All of the examples of Tables II and III were carried out insubstantially the same manner except as indicated in Tables II and III,and except as indicated herein.

Carbon content of the sintered body was determined by a standardchemical analysis technique.

Based on the predetermined oxygen content of the starting AlN powdersand the measured compositions of the resulting sintered bodies, as wellas other experiments, it was calculated or estimated that in everyexample in Tables II and III, the aluminum nitride in the compact beforedeoxidation had an oxygen content of about 0.3% by weight higher thanthat of the starting aluminum nitride powder.

Measured oxygen content was determined by neutron activation analysisand is given in wt %, which is % by weight of the sintered body.

In Tables II and III, in those examples where the oxygen content of thesintered body was measured, the equivalent % composition of the sinteredbody was calculated from the starting powder composition and from thegiven measured oxygen content of the sintered body. The Y, Al, N andoxygen are assumed to have their conventional valences of: +3, +3, -3,-2, respectively. In the sintered bodies, the equivalent percent amountof Y and Al was assumed to be the same as that in the starting powder.During processing, the amount of oxygen gain and nitrogen loss wasassumed to have occurred by the overall reaction:

    2 AlN+3/2O.sub.2 →Al.sub.2 O.sub.3 +N.sub.2         (13)

During deoxidation, the amount of oxygen loss and nitrogen gain wasassumed to have occurred by the overall reaction:

    Al.sub.2 O.sub.3 +3C+N.sub.2 →2AlN+3CO              (14)

The nitrogen content of the sintered body was determined by knowing theinitial oxygen content of the starting aluminum nitride powder andmeasuring the oxygen content of the sintered body and assuming thatreactions 13 and 14 have occurred.

In Tables II and III, an approximation sign is used in front of theequivalent percent oxygen for sintered bodies whose oxygen content wasnot measured. Since examples having the same number but including theletter A or B were carried out under the same conditions to produce thegiven pair of sintered bodies simultaneously, this pair of sinteredbodies will have the same oxygen content, and therefore, the oxygencontent of one such sintered body is assumed to be the same as themeasured oxygen content of the other such sintered body. Also, in TablesII and III, the equivalent % oxygen content of the sintered body ofExample 2 (Sample 108D) is assumed not to differ significantly from theequivalent % oxygen content of the sintered body of Example 1B (Sample108A2) and the equivalent % oxygen content of the sintered body ofExample 11 (Sample 175B) is assumed not to differ significantly from theequivalent % oxygen content of the sintered body of Example 10 (175A).The equivalent % oxygen content of the sintered bodies of Example 3(Sample 94C), Example 5 (Sample 150B) and Example 10 (Sample 175A) wascalculated from the X-ray diffraction analysis data.

The equivalent % oxygen of Examples 12B (131D1), 13(168A), 14(162A) and15(169A) were calculated from the following equation: ##EQU3## whereO=equivalent % oxygen

Y=equivalent % yttrium ##EQU4##

The equivalent % oxygen in Example 8 (90K) is assumed to be the same asthe equivalent percent oxygen as in another experiment where the powdermixture had the same composition, which was carried out in argon, andwhere the oxygen content of the sintered body was measured. Theequivalent % oxygen in Example 6 (Sample 150C) is assumed to be the sameas in another experiment where the powder mixture had the samecomposition and where the equivalent % oxygen was calculated from theX-ray diffraction analysis data.

Weight loss in Tables II and III is the difference between the weight ofthe compact after die pressing and the resulting sintered body.

Density of the sintered body was determined by the Archimedes method.

Porosity in % by volume of the sintered body was determined by knowingthe theoretical density of the sintered body on the basis of itscomposition and comparing that to the density measured using thefollowing equation: ##EQU5##

Phase composition of the sintered body was determined by opticalmicroscopy and X-ray diffraction analysis, and each sintered body wascomprised in % by volume of the sintered body of aluminum nitride phaseand the given volume % of the given second phases. The X-ray diffractionanalysis for volume % of each second phase is accurate to about ±20% ofthe given value.

The thermal conductivity of the sintered body of Example 8 (90K) wasmeasured by laser flash at about 25° C.

The thermal conductivity of the sintered body of all of the remainingexamples was measured at 25° C. by a steady state heat-flow method usinga rodshaped sample ˜0.4 cm×0.4 cm×2.2 cm sectioned from the sinteredbody. This method was originally devised by A. Berget in 1888 and isdescribed in an article by G. A. Slack in the "Encyclopaedic Dictionaryof Physics", Ed. by J. Thewlis, Pergamon, Oxford, 1961. In thistechnique the sample is placed inside a high-vacuum chamber, heat issupplied at one end by an electrical heater, and the temperatures aremeasured with fine-wire thermocouples. The sample is surrounded by aguard cylinder. The absolute accuracy is about ±3% and the repeatabilityis about ±1%. As a comparison, the thermal conductivity of an Al₂ O₃single crystal was measured with a similar apparatus to be 0.44 W/cm°K.at about 22° C.

In Tables II and III, the size of the resulting sintered body is givenas A, B, C or D. The body of A size was in the form of a disk about 0.17inch in thickness and about 0.32 inch in diameter. The body of B sizewas also in the form of a disk with a thickness of about 0.27 inch and adiameter of about 0.50 inch. The body of C size was in the shape of abar measuring about 0.16 inch ×0.16 inch ×1.7 inches. The body of D sizewas in the form of a substrate, i.e. a thin piece of uniform thickness,or of no significant difference in thickness, having a diameter of about1.5 inch and a thickness of 0.042 inch.

In all of the examples of Tables II and III, the compacts were placed ona molybdenum plate and then given the heat treatment shown in Tables IIand III.

In all of the Examples of Tables II and III wherein the sintered bodywas of C size or of D size, the starting compact was separated from themolybdenum plate by a thin discontinuous layer of AlN powder.

The sintered body of Example 2 exhibited some non-flatness, i.e.exhibited some warping, and was subjected to a flattening treatment.Specifically, the sintered body produced in Example 2 was sandwichedbetween a pair of molybdenum plates. The sandwiched sintered body wasseparated from the molybdenum plates by a thin discontinuous coating ormonolayer of aluminum nitride powder which was just sufficient toprevent sticking of the sintered body to the plates during theflattening treatment period. The top molybdenum plate exerted a pressureof about 0.11 psi on the sintered body. The sandwiched sintered body washeated in nitrogen, i.e. the same atmosphere used to sinter it, to about1900° C. where it was held for about 1 hour and then furnace cooled toabout room temperature. The resulting sintered body was flat and was ofuniform thickness, i.e. its thickness did not differ significantly. Thisflat sintered body would be useful as a supporting substrate for asemiconductor such as a silicon chip.

EXAMPLE 1

0.932 grams of Y₂ O₃ powder and 0.237 grams of graphite powder wereadded to 17.01 grams of aluminum nitride powder and the mixture, alongwith aluminum nitride milling media, was immersed in non-aqueous heptanecontaining oleic acid in an amount of about 0.7% by weight of thealuminum nitride in a plastic jar and vibratory milled in the closed jarat room temperature for about 18 hours. The resulting dispersion wasdried in air under a heat lamp for about 20 minutes and during suchdrying, the aluminum nitride picked up oxygen from the air. Duringmilling, the mixture picked up 0.772 gram AlN due to wear of the AlNmilling media.

Equivalent portions of the resulting dried mixture were die pressedproducing compacts.

Two of the compacts were placed side by side on a molybdenum plate.

The compacts were heated in nitrogen to 1500° C. where they were heldfor 1/2 hour, then the temperature was raised to 1600° C. where it washeld for 1/2 hour, and then the temperature was raised to 1870° C. whereit was held for 1 hour.

This example is shown as Examples 1A and 1B in Table II. Specifically,one of the sintered bodies, Example 1B, had a measured oxygen content of1.75% by weight of the body of the sintered body. Also, it had a phasecomposition comprised of AlN and 4.6% by volume of the body of Y₄ Al₂O₉. Also, it had an equivalent % composition comprised of 3.10% O,(100%-3.10%) or 96.90% N, 1.88% Y and (100%-1.88%) or 98.12% Al.

The compact used in Example 2 was produced in Example 1. Specifically,in Example 2, one compact was heated to 1600° C. where it was held for 1hour and then the temperature was raised to 1900° C. where it was heldfor 1 hour.

In Example 3, one compact was heated to 1500° C. where it was held for1/2 hour, then the temperature was raised to 1600° C. where it was heldfor 1 hour and then it was raised to 1950° C. where it was held for 1hour.

Examples 4A, 4B, 5, 6, 7A, 7B, 9A, 9B, 10, 11, 13, 14 and 15 werecarried out in the same manner as Example 2 except as indicated hereinand except as shown in Tables II and III. Also, Examples 8, 12A and 12Bwere carried out in the same manner as Example 3 except as indicatedherein and except as shown in Tables II and III.

                                      TABLE II                                    __________________________________________________________________________                                                          Properties of                                                                 Sintered Body                  Powder Mixture                                                                         Heat Treatment                        Measured                       (wt %)   Deoxidation   + Sintering       Atmos-                                                                              Oxygen                                                                             Carbon             Ex.                                                                              Sample                                                                            AlN                                                                              Y.sub.2 O.sub.3                                                                  C  Temp (°C.)                                                                     Time (Hr)                                                                             Temp (°C.)                                                                     Time (Hr)                                                                             phere (wt                                                                                (wt                __________________________________________________________________________                                                               %)                                                                 1A    108A1                                                                              93.85 4.91 1.25                                                               51500                                                                    1/2  + 1870                                                                   1                                                                             N.sub.2                                                                            -- --                                                                            1600                                                                  1/2                                                                     1B    108A2                                                                              " " "  "    "                                                                 1" 1.75 --                                                         2     108D " " " 1600                                                               1    + 1900                                                                   1                                                                             N.sub.2                                                                            -- --                                                              3     94C  96.01 2.13 1.86                                                               71500                                                                    1/2  + 1950                                                                   1                                                                             N.sub.2                                                                            -- --                                                                            1600                                                                  1                                                                       4A    104A1                                                                              97.62 0.67 1.71                                                               .1600                                                                    1    + 2050                                                                   1                                                                             N.sub.2                                                                             0.355 --                                                          4B    104A2                                                                              " " "  "    "                                                                 7" -- 0.027                                                        5     150B 92.35 7.00 0.65                                                               21600                                                                    1    + 1870                                                                   1                                                                             N.sub.2                                                                            -- --                                                              6     150C " " " 1600                                                               1    + 1950                                                                   1                                                                             N.sub.2                                                                            -- --                                                              7A    84G  88.98 7.75 1.27                                                               N1600                                                                    1    + 1900                                                                   1                                                                             Ar   4.02 0.372                                                         7B    84G1 " " "  "    "                                                                 O                                                                        "    -- --                                                              8     90K  88.98 9.69 1.32                                                                1500                                                                    1/2  + 1900                                                                   1                                                                             Ar   -- --                                                                            1600                                                                  1                       __________________________________________________________________________    Properties of Sintered Body                                                                Weight   Approximate                                                                          Volume % of   Thermal                            Equivalent % Loss                                                                              Density                                                                            Porosity                                                                             Second Phases Conductivity                       Ex.                                                                              Oxygen                                                                             Yttrium                                                                            (%) (g/cc)                                                                             (vol %)                                                                              Y.sub.4 Al.sub.2 O.sub.9                                                           YAlO.sub.3                                                                         Y.sub.2 O.sub.3                                                                   (W/cm · K                                                            @ 25° C.)                                                                          Size                   __________________________________________________________________________    1A ˜3.10                                                                        1.88 --  3.20 4      --   --   --  --          A                      1B 3.10 1.88 4.0 --   --     4.6  --   --  --          A                      2  ˜3.1                                                                         1.88 --  --   --     --   --   --  --          D(.042" thick)         3  ˜1.8                                                                         0.81 --  3.27 <1     0.9  0.5  --  1.64        C                      4A 0.61 0.25 5.9 2.90 11     --   --   --  --          A                      4B ˜0.61                                                                        0.25 6.7 2.90 11     0.5  --   --  --          A                      5  ˜5.2                                                                         2.70 --  3.36 <1     3.7  3.1  --  1.51        C                      6  ˜5.0                                                                         2.70 --  3.35 <1     --   --   --  1.56        C                      7A 7.39 3.86 5.5 3.31 2      --   --   --  --          A                      7B ˜7.39                                                                        3.86 5.6 --   --     7.1  --   0.5 --          A                      8  ˜7.1                                                                         3.84 4.7 3.26 4      8.5  --   --  ˜0.48 B                      __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________                           Heat Treatment                                                Powders Added (wt %)                                                                          Deoxidation   + Sintering                              Ex.                                                                              Sample                                                                            AlN   Y.sub.2 O.sub.3                                                                     C   Temp °C.                                                                       Time Hr Temp °C.                                                                       Time Hr                                                                               Atmosphere             __________________________________________________________________________                                                           9A 98A1 94.19 4.75                                                            1.06 1600                                                                      1 + 1860                                                                      1                                                                             N.sub.2                                                                      9B 98A2 " " "  "  +                                                             "                                                                            "                                                                            10 175A 97.19-(0.5)                                                           * 1.71-(0.6)* 1.10*                                                           ** 1600                                                                        1 + 1900                                                                      1                                                                             N.sub.2                                                                      11 175B " " " 1600                                                            N                                                                              1 + 2000                                                                      1                                                                             N.sub.2                                                                      12A 131D 89.42 9.39                                                           O1.19 1500                                                                     1/2 + 1900                                                                    1                                                                             H.sub.2 + 25%                                                                N.sub.2                                                                            1600                                                                      1                                                                            12B 131D1 " " "  "                                                             +  "                                                                          "                                                                            13 168A 89.74-(0.5)                                                           * 9.68** 0.57 1600                                                             1 + 1900                                                                      1                                                                             N.sub.2                                                                      14 162A 89.62-(1.6)                                                           * 9.65** 0.73 1600                                                             1 + 1900                                                                      1                                                                             N.sub.2                                                                      15 169A 89.72-(0.5)                                                           * 9.68-(0.6)* 0.60*                                                           ** 1600                                                                        1 + 1900                                                                      1                                                                             N.sub.2               __________________________________________________________________________    Properties of Sintered Body                                                   Measured              Wt      Approximate                                                                          Volume      Thermal                      Oxygen  Carbon                                                                            Equivalent %                                                                            Loss                                                                             Density                                                                            Porosity                                                                             % Second Phases                                                                           Conductivity                 Ex.                                                                              (wt %)                                                                             (wt %)                                                                            Oxygen                                                                             Yttrium                                                                            %  (g/cc)                                                                             (%)    Y.sub.2 O.sub.3                                                                  Y.sub.4 Al.sub.2 O.sub.9                                                           YAlO.sub.3                                                                        W/cm · K @                                                           25° C.                                                                           Size               __________________________________________________________________________    9A --   --  ˜3.93                                                                        1.81 3.8                                                                              3.35 <1     -- --   --  --        A                  9B 2.22 0.019                                                                             3.93 1.81 3.8                                                                              "    --     -- 0.2  3.5 --        A                  10 --   --  ˜1.3                                                                         0.64 -- 2.54 23     -- 1.3  --  --        C                  11 --   --  ˜1.3                                                                         0.64 -- 2.56 22     -- --   --  --        C                  12A                                                                              --   --  ˜6.0                                                                         3.70 -- 3.35 1      -- --   --  1.52      C                  12B                                                                              --   0.014                                                                             ˜6.0                                                                         3.70 2.8                                                                              --   --     1.2                                                                              5.8  --  --        A                  13 --   --  ˜5.6                                                                         3.79 -- 3.09 9      2.6                                                                              4.5  --  1.29      C                  14 --   --  ˜5.5                                                                         3.79 -- 3.30 2      2.6                                                                              4.1  --  --        C                  15 --   --  ˜6.0                                                                         3.80 -- 3.37 <1     2.0                                                                              6.9  --  1.41      C                  __________________________________________________________________________     *Specific surface area of added powder in m.sup.2 /g is given in              parenthesis                                                                   **Y.sub.2 (CO.sub.3).sub.3 .3H.sub.2 O added as source of Y.sub.2 O.sub.3     ***Specific surface area of carbon = 25 m.sup.2 /g                       

Examples 1A, 1B, 2, 3, 9A and 9B illustrate the present invention. Thesintered body produced in Examples 1A, 1B, 2, 3, 9A and 9B is useful forpackaging of integrated circuits as well as for use as a substrate orcarrier for a semiconductor such as a silicon chip.

Examples 1A and 1B illustrate the present invention, and have acomposition which is on line P1J of FIG. 4. Knowing that the thermalconductivity of the AlN sintered body decreases with increasing contentof second phase, and based on other experiments and a comparison ofExamples 1A and 1B with Example 5 where the sintered body containedsignificantly more second phase, it is known that the sintered bodyproduced in Examples 1A and 1B had a thermal conductivity greater than1.42 W/cm.K at 25° C.

Example 2 illustrates the present invention. Based on a comparison ofExample 2 with Examples 1A and 1B which have the same powder mixture,and based on other experiments, it is known that the sintered body ofExample 2 had a composition which was the same or which did not differsignificantly from that of the sintered bodies of Examples 1A and 1B,Specifically, the sintered body produced in Example 2 was comprised ofAlN phase and about 4.6% by volume of the sintered body of Y₄ Al₂ O₉phase, and has a composition which is on about line P1J of FIG. 4. Also,knowing that the thermal conductivity of the AlN sintered body decreaseswith increasing content of second phase, and based on other experimentsand a comparison of Example 2 with Example 5 where the sintered bodycontained significantly more second phase, it is known that the sinteredbody produced in Example 2 had a thermal conductivity greater than 1.42W/cm.K at 25° C.

The sintered body produced in Example 3, which illustrates the presentinvention, has a composition defined and encompassed by polygon P1JFA4excluding lines JF and A4F of FIG. 4.

Examples 9A and 9B illustrate the present invention, and have acomposition defined and encompassed by polygon P1JFA4 excluding lines JFand A4F of FIG. 4. Knowing that the thermal conductivity of the AlNsintered body decreases with increasing content of second phase, andbased on other experiments and a comparison of Example 9A and withExample 5 where the sintered body contained significantly more secondphase, it is known that the sintered body produced in Examples 9A and 9Bhad a thermal conductivity greater than 1.42 W/cm.K at 25° C.

The powder mixtures of Examples 4A and 4B contained less than 0.3equivalent % yttrium. The equivalent % composition of the sinteredbodies of Examples 4A and 4B fell outside polygon P1JFA4 of FIG. 4 andspecifically they fell below point P1 of FIG. 4. In Examples 4A and 4B,the sintered bodies had a porosity higher than 10% by volume of the bodywhich illustrates the difficulty of sintering in this composition areabelow point P1 of FIG. 4.

In Examples 5 and 6, the sintered bodies had a composition outsidepolygon P1JFA4 of FIG. 4, and more specifically, above line JF.

Examples 7A and 7B illustrate that even though there was a deoxidationof the compact, the use of the argon atmosphere resulted in a largeamount of carbon being left in the sintered body.

Example 8 illustrates that the use of an argon atmosphere results in asintered body having a low thermal conductivity.

Examples 10 and 11 illustrate that the minimum sintering temperatureincreases with an increase in particle size of AlN. Specifically, at thecomposition ˜1.3 equivalent % oxygen and 0.64 equivalent % yttrium, itis difficult to sinter a compact, even at 2000° C., prepared from theparticle size combination of AlN, Y₂ O₃ and carbon of about 0.5, m² /g,0.6 m² /g, and 25 m² /g, respectively.

In U.S. Pat. Nos. 4,478,785 and 4,533,645 entitled HIGH THERMALCONDUCTIVITY ALUMINUM NITRIDE CERAMIC BODY, incorporated herein byreference, there is disclosed the process comprising forming a mixturecomprised of aluminum nitride powder and free carbon wherein thealuminum nitride has a predetermined oxygen content higher than about0.8% by weight and wherein the amount of free carbon reacts with suchoxygen content to produce a deoxidized powder or compact having anoxygen content ranging from greater than about 0.35% by weight to about1.1% by weight and which is at least 20% by weight lower than thepredetermined oxygen content, heating the mixture or a compact thereofto react the carbon and oxygen producing the deoxidized aluminumnitride, and sintering a compact of the deoxidized aluminum nitrideproducing a ceramic body having a density greater than 85% oftheoretical and a thermal conductivity greater than 0.5 W/cm.K at 22° C.

In copending U.S. patent application Ser. No. 656,636, entitled HIGHTHERMAL CONDUCTIVITY CERAMIC BODY, filed on Oct. 1, 1984, in the namesof Irvin Charles Huseby and Carl Francis Bobik and assigned to theassignee hereof and incorporated herein by reference, there is disclosedthe process for producing an aluminum nitride ceramic body having acomposition defined and encompassed by polygon JKLM but not includingline MJ of FIG. 4 of Ser. No. 656,636 and a thermal conductivity greaterthan 1.42 W/cm.K at 25° C. which comprises forming a mixture comprisedof aluminum nitride powder containing oxygen, yttrium oxide, and freecarbon, shaping said mixture into a compact, said mixture and saidcompact having a composition wherein the equivalent % of yttrium andaluminum ranges from point L to less than point J of FIG. 4 of Ser. No.656,636, said compact having an equivalent % composition of Y, Al, O andN outside the composition defined and encompassed by polygon JKLM ofFIG. 4 of Ser. No. 656,636, the aluminum nitride in said compactcontaining oxygen in an amount ranging from greater than about 1.4% byweight to less than about 4.5% by weight of the aluminum nitride,heating said compact up to a temperature at which its pores remain openreacting said free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon JKLM but not including line MJ of FIG. 4 of Ser.No. 656,636, and sintering said deoxidized compact at a temperatureranging from about 1890° C. to about 2050° C. producing said ceramicbody.

In copending U.S. patent application Ser. No. 667,516 entitled HIGHTHERMAL CONDUCTIVITY CERAMIC BODY, filed Nov. 1, 1984, in the names ofIrvin Charles Huseby and Carl Francis Bobik and assigned to the assigneehereof and incorporated herein by reference, there is disclosed theprocess for producing an aluminum nitride ceramic body having acomposition defined and encompassed by polygon FJDSR but not includingline RF of FIG. 4 of Ser. No. 667,516, a porosity of less than about 4%by volume, and a thermal conductivity greater than 1.25 W/cm.K at 25° C.which comprises forming a mixture comprised of aluminum nitride powdercontaining oxygen, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges from point D upto point F of FIG. 4 of Ser. No. 667,516, said compact having anequivalent % composition of Y, Al, O and N outside the compositiondefined and encompassed by polygon FJDSR of FIG. 4 of Ser. No. 667,516,the aluminum nitride in said compact containing oxygen in an amountranging from greater than about 1.95% by weight to less than about 5.1%by weight of the aluminum nitride, heating said compact up to atemperature at which its pores remain open reacting said free carbonwith oxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonFJDSR but not including line RF of FIG. 4 of Ser. No. 667,516, andsintering said deoxidized compact at a temperature ranging from about1870° C. to about 2050° C. producing said ceramic body.

In copending U.S. patent application Ser. No. 675,048 entitled HIGHTHERMAL CONDUCTIVITY CERAMIC BODY filed about Nov. 26, 1984 in the namesof Irvin Charles Huseby and Carl Francis Bobik and assigned to theassignee hereof and incorporated herein by reference, there is disclosedthe process for producing an aluminum nitride ceramic body having acomposition defined and encompassed by polygon PONKJ but not includinglines KJ and PJ of FIG. 4 of Ser. No. 675,048, a porosity of less thanabout 4% by volume, and a minimum thermal conductivity of 1.50 W/cm.K at25° C. which comprises forming a mixture comprised of aluminum nitridepowder containing oxygen, yttrium oxide, and free carbon, shaping saidmixture into a compact, said mixture and said compact having acomposition wherein the equivalent % of yttrium and aluminum rangesbetween points K and P of FIG. 4 of Ser. No. 675,048, said compacthaving an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon PONKJ of FIG. 4 of Ser.No. 675,048, the aluminum nitride in said compact containing oxygen inan amount ranging from greater than about 1.40% by weight to less thanabout 4.50% by weight of the aluminum nitride, heating said compact upto a temperature at which its pores remain open reacting said freecarbon with oxygen contained in said aluminum nitride producing adeoxidized compact, said deoxidized compact having a composition whereinthe equivalent % of Al, Y, O and N is defined and encompassed by polygonPONKJ but not including lines KJ and PJ of FIG. 4 of Ser. No. 675,048,and sintering said deoxidized compact at a temperature ranging fromabout 1900° C. to about 2050° C. producing said ceramic body, saidsintering temperature being a sintering temperature for said compositionof said deoxidized compact.

In copending U.S. patent application Ser. No. 667,516 entitled HIGHTHERMAL CONDUCTIVITY CERAMIC BODY, filed Nov. 1, 1984, in the names ofIrvin Charles Huseby and Carl Francis Bobik and assigned to the assigneehere of and incorporated herein by reference, there is disclosed theprocess for producing an aluminum nitride ceramic body having acomposition defined and encompassed by polygon FJDSR but not includingline RF of FIG. 4 of Ser. No. 667,516, a porosity of less than about 4%by volume, and a thermal conductivity greater than 1.25 W/cm.K at 25° C.which comprises forming a mixture comprised of aluminum nitride powdercontaining oxygen, yttrium oxide, and free carbon, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges from point D upto point F of FIG. 4 of Ser. No. 667,516, said compact having anequivalent % composition of Y, Al, O and N outside the compositiondefined and encompassed by polygon FJDSR of FIG. 4 of Ser. No. 667,516,the aluminum nitride in said compact containing oxygen in an amountranging from greater than about 1.95% by weight to less than about 5.1%by weight of the aluminum nitride, heating said compact up to atemperature at which its pores remain open reacting said free carbonwith oxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonFJDSR but not including line RF of FIG. 4 of Ser. No. 667,516, andsintering said deoxidized compact at a temperature ranging from about1870° C. to about 2050° C. producing said ceramic body.

In copending U.S. patent application Ser. No. 682,468, entitled HIGHTHERMAL CONDUCTIVITY CERAMIC BODY, filed on Dec. 17, 1984, in the namesof Irvin Charles Huseby and Carl Francis Bobik and assigned to theassignee herein and incorporated herein by reference, there is discloseda process for producing an aluminum nitride ceramic body having acomposition defined and encompassed by polygon LT1DM but not includinglines LM and DM of FIG. 4 of Ser. No. 682,468, a porosity of less thanabout 4% by volume, and a minimum thermal conductivity of 1.27 W/cm.K at25° C. which comprises forming a mixture comprised of aluminum nitridepowder containing oxygen, yttrium oxide, and free carbon, shaping saidmixture into a compact, said mixture and said compact having acomposition wherein the equivalent % of yttrium and aluminum ranges frompoint T1 up to point M of FIG. 4 of Ser. No. 682,468, said compacthaving an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon LT1DM of FIG. 4 of Ser.No. 682,468, the aluminum nitride in said compact containing oxygen inan amount ranging from greater than about 1.85% by weight to less thanabout 4.50% by weight of the aluminum nitride, heating said compact upto a temperature at which its pores remain open reacting said freecarbon with oxygen contained in said aluminum nitride producing adeoxidized compact, said deoxidized compact having a composition whereinthe equivalent % of Al, Y, O and N is defined and encompassed by polygonLT1DM but not including lines LM and DM of FIG. 4 of Ser. No. 862,468,and sintering said deoxidized compact at a temperature ranging fromabout 1890° C. to about 2050° C. wherein the minimum sinteringtemperature increases from about 1890° C. for a composition adjacent toline DM to about 1970° C. for a composition on line T1L producing saidceramic body, said sintering temperature being a sintering temperaturefor said composition of said deoxidized compact.

What is claimed is:
 1. A process for producing a sinteredpolycrystalline aluminum nitride ceramic body having a compositiondefined and encompassed by polygon P1JFA4 but not including lines JF andA4F of FIG. 4, a porosity of less than about 10% by volume of said bodyand a thermal conductivity greater than 1.00 W/cm.K at 25° C. whichcomprises the steps:(a) forming a mixture comprised of oxygen-containingaluminum nitride powder, yttrium oxide, and free carbon, shaping saidmixture into a compact, said mixture and said compact having acomposition wherein the equivalent % of yttrium and aluminum rangesbetween points J and A4 of FIG. 4, said yttrium ranging from greaterthan about 0.3 equivalent % to less than about 2.5 equivalent %, saidaluminum ranging from greater than about 97.5 equivalent % to less thanabout 99.7 equivalent %, said compact having an equivalent % compositionof Y, Al, O and N outside the composition defined and encompassed bypolygon P1JFA4 of FIG. 4, (b) heating said compact in anitrogen-containing nonoxidizing atmosphere at a temperature rangingfrom about 1350° C. to a temperature sufficient to deoxidize the compactbut below its pore closing temperature reacting said free carbon withoxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonP1JFA4 but not including lines JF and A4F of FIG. 4, said free carbonbeing in an amount which produces said deoxidized compact, and (c)sintering said deoxidized compact in a nitrogen-containing nonoxidizingatmosphere at a temperature of at least about 1850° C. producing saidpolycrystalline body.
 2. The process according to claim 1 wherein saidnitrogen-containing atmosphere in step (b) contains sufficient nitrogento facilitate deoxidation of the aluminum nitride to produce saidsintered body.
 3. The process according to claim 1 wherein saidnitrogen-containing atmosphere in step (c) contains sufficient nitrogento prevent significant weight loss of said aluminum nitride.
 4. Theprocess according to claim 1 wherein said process is carried out atambient pressure.
 5. The process according to claim 1 wherein thealuminum nitride in said compact in step (a) before said deoxidation ofstep (b) contains oxygen in an amount ranging from greater than about1.0% by weight to less than about 4.7% by weight of said aluminumnitride.
 6. The process according to claim 1 wherein said aluminumnitride in step (a) has a specific surface area ranging up to about 10m² /g and said free carbon has a specific surface area greater thanabout 10 m² /g.
 7. The process according to claim 1 wherein said mixtureand said compact have a composition wherein the equivalent % of yttriumand aluminum ranges between points J and A2 of FIG. 4, said yttriumranging from greater than about 0.65 equivalent % to less than about 2.5equivalent %, said aluminum ranging from greater than about 97.5equivalent % to less than about 99.35 equivalent %, and wherein saidsintered body and said deoxidized compact are comprised of a compositionwherein the equivalent percent of Al, Y, O and N is defined andencompassed by polygon A3JFA2 but does not include lines A3J, JF and A2Fof FIG.
 4. 8. The process according to claim 1 wherein said mixture andsaid compact have a composition wherein the equivalent % of yttrium andaluminum ranges from less than point A3 up to point A4 of FIG. 4, saidyttrium ranging from about 0.3 equivalent % to less than about 0.85equivalent %, said aluminum ranging from greater than about 99.15equivalent % to less than about 99.7 equivalent %, and wherein saidsintered body and said deoxidized compact are comprised of a compositionwherein the equivalent percent of Al, Y, O and N is defined andencompassed by polygon P1A3A2A4 but does not include lines P1A3, A3A2and A2A4 of FIG.
 4. 9. The process according to claim 1 wherein saidmixture and said compact have a composition wherein the equivalent % ofyttrium and aluminum ranges from point P1 to point A3 of FIG. 4, saidyttrium ranging from about 0.35 equivalent % to about 0.85 equivalent %,said aluminum ranging from about 99.15 equivalent % to about 99.65equivalent %, and wherein said sintered body and said deoxidized compactare comprised of a composition wherein the equivalent percent of Al, Y,O and N is defined by line P1A3 of FIG. 4, and said sinteringtemperature is at least about 1860° C.
 10. The process according toclaim 1 wherein said mixture and said compact have a composition whereinthe equivalent % of yttrium and aluminum ranges from point A3 up topoint J of FIG. 4, said yttrium ranging from about 0.85 equivalent % toless than about 2.5 equivalent %, said aluminum ranging from greaterthan about 97.5 equivalent % to about 99.15 equivalent %, and whereinsaid sintered body and said deoxidized compact are comprised of acomposition wherein the equivalent percent of Al, Y, O and N is definedby line A3J but not including point J of FIG. 4, and said sinteringtemperature is at least about 1860° C.
 11. A process for producing asintered polycrystalline aluminum nitride ceramic body having acomposition defined and encompassed by polygon A3JFA2 but not includinglines A3J, JF and A2F of FIG. 4, a porosity of less than about 10% byvolume of said body and a thermal conductivity greater than 1.00 W/cm.Kat 25° C. which comprises the steps:(a) forming a mixture comprised ofan oxygen-containing aluminum nitride powder, yttrium oxide, and freecarbon, said free carbon having a specific surface area greater thanabout 100 m² /g, the aluminum nitride powder in said mixture having aspecific surface area ranging from about 3.4 m² /g to about 6.0 m² /g,shaping said mixture into a compact, said mixture and said compacthaving a composition wherein the equivalent % of yttrium and aluminumranges between points J and A2 of FIG. 4, said yttrium ranging fromgreater than about 0.65 equivalent % to less than about 2.5 equivalent%, said aluminum ranging from greater than about 97.5 equivalent % toless than about 99.35 equivalent %, said compact having an equivalent %composition of Y, Al, O and N outside the composition defined andencompassed by polygon P1JFA4 of FIG. 4, the aluminum nitride in saidcompact containing oxygen in an amount ranging from greater than about1.42% by weight to less than about 4.70% by weight of said aluminumnitride, (b) heating said compact at ambient pressure in anitrogen-containing nonoxidizing atmosphere containing at least about25% by volume nitrogen at a temperature ranging from about 1350° C. to atemperature sufficient to deoxidize the compact but below its poreclosing temperature reacting said free carbon with oxygen contained insaid aluminum nitride producing a deoxidized compact, said deoxidizedcompact having a composition wherein the equivalent % of Al, Y, O and Nis defined and encompassed by polygon A3JFA2 but not including linesA3J, JF and A2F of FIG. 4, the aluminum nitride in said compact beforesaid deoxidation by said carbon having an oxygen content ranging fromgreater than about 1.42% by weight to less than about 4.70% by weight ofsaid aluminum nitride, said free carbon being in an amount whichproduces said deoxidized compact, and (c) sintering said deoxidizedcompact at ambient pressure in a nitrogen-containing nonoxidizingatmosphere containing at least about 25% by volume nitrogen at atemperature ranging from about 1885° C. to about 1970° C. producing saidpolycrystalline body.
 12. The process according to claim 11 wherein thesintering temperature ranges from about 1890° C. to about 1950° C., saidaluminum nitride powder in said mixture has a specific surface arearanging from about 3.5 m² /g to about 6.0 m² /g, and said sintered bodyhas a porosity of less than about 1% by volume of said body.
 13. Theprocess according to claim 11 wherein the sintering temperature rangesfrom about 1940° C. to about 1970° C., and said sintered body containscarbon in an amount of less than about 0.04% by weight of said body andhas a porosity of less than about 1% by volume of said body and athermal conductivity greater than about 1.57 W/cm.K at 25° C.
 14. Aprocess for producing a sintered polycrystalline aluminum nitrideceramic body having a composition defined and encompassed by polygonP1JFA4 but not including lines JF and A4F of FIG. 4, a porosity of lessthan about 10% by volume of said body and a thermal conductivity greaterthan 1.00 W/cm.K at 25° C. which comprises the steps:(a) forming amixture comprised of an oxygen-containing aluminum nitride powder,yttrium oxide or precursor therefor, and a carbonaceous additiveselected from the group consisting of free carbon, a carbonaceousorganic material and mixtures thereof, said carbonaceous organicmaterial thermally decomposing at a temperature ranging from about 50°C. to about 1000° C. to free carbon and gaseous product of decompositionwhich vaporizes away, shaping said mixture into a compact, said mixtureand said compact having a composition wherein the equivalent % ofyttrium and aluminum ranges between points J and A4 of FIG. 4, saidyttrium ranging from greater than about 0.3 equivalent % to less thanabout 2.5 equivalent %, said aluminum ranging from greater than about97.5 equivalent % to less than about 99.7 equivalent % aluminum, saidcompact having an equivalent % composition of Y, Al, O and N outside thecomposition defined and encompassed by polygon P1JFA4 of FIG. 4, (b)heating said compact in a nonoxidizing atmosphere at a temperature up toabout 1200° C. thereby providing yttrium oxide and free carbon, (c)heating said compact in a nitrogen-containing nonoxidizing atmosphere ata temperature ranging from about 1350° C. to a temperature sufficient todeoxidize the compact but below its pore closing temperature reactingsaid free carbon with oxygen contained in said aluminum nitrideproducing a deoxidized compact, said deoxidized compact having acomposition wherein the equivalent % of Al, Y, O and N is defined andencompassed by polygon P1JFA4 but not including lines JF and A4F of FIG.4, said free carbon being in an amount which produces said deoxidizedcompact, and (d) sintering said deoxidized compact in anitrogen-containing nonoxidizing atmosphere at a temperature of at leastabout 1850° C. producing said polycrystalline body.
 15. The processaccording to claim 14 wherein said nitrogen-containing atmosphere instep (c) contains sufficient nitrogen to facilitate deoxidation of thealuminum nitride to produce said sintered body.
 16. The processaccording to claim 14 wherein said nitrogen-containing atmosphere instep (d) contains sufficient nitrogen to prevent significant weight lossof said aluminum nitride.
 17. The process according to claim 14 whereinsaid process is carried out at ambient pressure.
 18. The processaccording to claim 14 wherein the aluminum nitride in said compact instep (a) before said deoxidation of step (c) contains oxygen in anamount ranging from greater than about 1.0% by weight to less than about4.7% by weight of said aluminum nitride.
 19. The process according toclaim 14 wherein said aluminum nitride in step (a) has a specificsurface area ranging up to about 10 m² /g and said free carbon has aspecific surface area greater than about 10 m² /g.
 20. The processaccording to claim 14 wherein said mixture and said compact have acomposition wherein the equivalent % of yttrium and aluminum rangesbetween points J and A2 of FIG. 4, said yttrium ranging from greaterthan about 0.65 equivalent % to less than about 2.5 equivalent %, saidaluminum ranging from greater than about 97.5 equivalent % to less thanabout 99.35 equivalent %, and wherein said sintered body and saiddeoxidized compact are comprised of a composition wherein the equivalentpercent of Al, Y, O and N is defined and encompassed by polygon A3JFA2but does not include lines A3J, JF and A2F of FIG.
 4. 21. The processaccording to claim 14 wherein said mixture and said compact have acomposition wherein the equivalent % of yttrium and aluminum ranges fromless than point A3 up to point A4 of FIG. 4, said yttrium ranging fromgreater than about 0.3 equivalent % to about 0.85 equivalent %, saidaluminum ranging from about 99.15 equivalent % to less than about 99.7equivalent %, and wherein said sintered body and said deoxidized compactare comprised of a composition wherein the equivalent percent of Al, Y,O and N is defined and encompassed by polygon P1A3A2A4 but does notinclude lines P1A3, A3A2 and A2A4 of FIG.
 4. 22. The process accordingto claim 14 wherein said mixture and said compact have a compositionwherein the equivalent % of yttrium and aluminum ranges from point P1 topoint A3 of FIG. 4, said yttrium ranging from about 0.35 equivalent % toabout 0.85 equivalent %, said aluminum ranging from about 99.15equivalent % to about 99.65 equivalent %, and wherein said sintered bodyand said deoxidized compact are comprised of a composition wherein theequivalent percent of Al, Y, O and N is defined by line P1A3 of FIG. 4,and said sintering temperature is at least about 1860° C.
 23. Theprocess according to claim 14 wherein said mixture and said compact havea composition wherein the equivalent % of yttrium and aluminum rangesfrom point A3 up to point J of FIG. 4, said yttrium ranging from about0.85 equivalent % to less than about 2.5 equivalent %, said aluminumranging from greater than about 97.5 equivalent % to about 99.15equivalent %, and wherein said sintered body and said deoxidized compactare comprised of a composition wherein the equivalent percent of Al, Y,O and N is defined by line A3J but not including point J of FIG. 4, andsaid sintering temperature is at least about 1860° C.
 24. A process forproducing a sintered polycrystalline aluminum nitride ceramic bodyhaving a composition defined and encompassed by polygon A3JFA2 but notincluding lines A3J, JF and A2F of FIG. 4, a porosity of less than about10% by volume of said body and a thermal conductivity greater than 1.00W/cm.K at 25° C. which comprises the steps:(a) forming a mixturecomprised of aluminum nitride powder, yttrium oxide or precursortherefor, and a carbonaceous additive selected from the group consistingof free carbon, a carbonaceous organic material and mixtures thereof,said carbonaceous organic material thermally decomposing at atemperature ranging from about 50° C. to about 1000° C. to free carbonand gaseous product of decomposition which vaporizes away, said freecarbon having a specific surface area greater than about 100 m² /g, thealuminum nitride powder in said mixture having a specific surface arearanging from about 3.4 m² /g to about 6.0 m² /g, shaping said mixtureinto a compact, said mixture and said compact having a compositionwherein the equivalent % of yttrium and aluminum ranges between points Jand A2 of FIG. 4, said yttrium ranging from greater than about 0.65equivalent % to less than about 2.5 equivalent %, said aluminum rangingfrom greater than about 97.5% equivalent % to less than about 99.35equivalent %, said compact having an equivalent % composition of Y, Al,O and N outside the composition defined and encompassed by polygonP1JFA4 of FIG. 4, the aluminum nitride in said compact containing oxygenin an amount ranging from greater than about 1.42% by weight to lessthan about 4.70% by weight of said aluminum nitride, (b) heating saidcompact in a nonoxidizing atmosphere at a temperature up to about 1200°C. thereby providing yttrium oxide and free carbon, (c) heating saidcompact at ambient pressure in a nitrogen-containing atmospherecontaining at least about 25% by volume nitrogen at a temperatureranging from about 1350° C. to a temperature sufficient to deoxidize thecompact but below its pore closing temperature reacting said free carbonwith oxygen contained in said aluminum nitride producing a deoxidizedcompact, said deoxidized compact having a composition wherein theequivalent % of Al, Y, O and N is defined and encompassed by polygonA3JFA2 but not including lines A3J, JF and A2F of FIG. 4, the aluminumnitride in said compact before said deoxidation by said carbon having anoxygen content ranging from greater than about 1.42% by weight to lessthan about 4.70% by weight of said aluminum nitride, said free carbonbeing in an amount which produces said deoxidized compact, and (d)sintering said deoxidized compact at ambient pressure in anitrogen-containing nonoxidizing atmosphere containing at least about25% by volume nitrogen at a temperature ranging from about 1885° C. toabout 1970° C. producing said polycrystalline body.
 25. The processaccording to claim 24 wherein said sintering temperature ranges fromabout 1890° C. to about 1950° C., said aluminum nitride powder in saidmixture has a specific surface area ranging from about 3.5 m² /g toabout 6.0 m² /g, and said sintered body has a porosity of less thanabout 1% by volume of said body.
 26. The process according to claim 24wherein said sintering temperature ranges from about 1940° C. to about1970° C., and said sintered body contains carbon in an amount of lessthan about 0.04% by weight of said body and has a porosity of less thanabout 1% by volume of said body and a thermal conductivity greater thanabout 1.57 W/cm.K at 25° C.
 27. A polycrystalline body having acomposition defined and encompassed by polygon P1JFA4 of FIG. 4 butexcluding lines JF and A4F which is comprised of from greater than about0.3 equivalent % yttrium to less than about 2.5 equivalent % yttrium,from greater than about 97.5 equivalent % aluminum to less than about99.7 equivalent % aluminum, from about 0.85 equivalent % oxygen to lessthan about 4.1 equivalent % oxygen and from greater than about 95.9equivalent % nitrogen to about 99.15 equivalent % nitrogen, saidpolycrystalline body having a porosity of less than about 10% by volumeof said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.
 28. A polycrystalline body having a composition defined andencompassed by polygon A3JFA2 of FIG. 4 but excluding lines A3J, JF andA2F which is comprised of from greater than about 0.65 equivalent %yttrium to less than about 2.5 equivalent % yttrium, from greater thanabout 97.5 equivalent % aluminum up to about 99.35 equivalent %aluminum, from about 1.6 equivalent % oxygen to less than about 4.1equivalent % oxygen and from greater than about 95.9 equivalent %nitrogen to about 98.4 equivalent % nitrogen, said polycrystalline bodyhaving a porosity of less than about 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C.
 29. Apolycrystalline body having a composition defined and encompassed bypolygon P1A3A2A4 of FIG. 4 but excluding lines P1A3, A3A2 and A2A4 whichis comprised, of from greater than about 0.3 equivalent % yttrium toless than about 0.85 equivalent % yttrium, from greater than about 99.15equivalent % aluminum to less than about 99.7 equivalent % aluminum,from greater than about 0.85 equivalent % oxygen to less than about 2.1equivalent % oxygen and from greater than about 97.9 equivalent %nitrogen to less than about 99.15 equivalent % nitrogen, saidpolycrystalline body having a porosity of less than about 10% by volumeof said body and a thermal conductivity greater than 1.00 W/cm.K at 25°C.
 30. A polycrystalline body having a composition defined by line P1A3of FIG. 4 which is comprised of from about 0.35 equivalent % yttrium toabout 0.85 equivalent % yttrium, from about 99.15 equivalent % aluminumto about 99.65 equivalent % aluminum, from about 0.85 equivalent %oxygen to about 1.6 equivalent % oxygen and from about 98.4 equivalent %nitrogen to about 99.15 equivalent % nitrogen, said polycrystalline bodyhaving a porosity of less than about 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C.
 31. Apolycrystalline body having a composition defined by line A3J of FIG. 4but excluding point J which is comprised of from about 0.85 equivalent %yttrium to less than about 2.5 equivalent % yttrium, from greater thanabout 97.5 equivalent % aluminum to about 99.15 equivalent % aluminum,from about 1.6 equivalent % oxygen to less than about 4.1 equivalent %oxygen and from greater than about 95.9 equivalent % nitrogen to about98.4 equivalent % nitrogen, said polycrystalline body having a porosityof less than about 10% by volume of said body and a thermal conductivitygreater than 1.00 W/cm.K at 25° C.
 32. A polycrystalline body having aphase composition comprised of AlN, YAlO₃ and Y₄ Al₂ O₉ wherein thetotal amount of said YAlO₃ and Y₄ Al₂ O₉ phases ranges from greater thanabout 0.8% by volume to less than about 6.0% by volume of the totalvolume of said body, said YAlO₃ phase ranging from a trace amount toless than about 4.2% by volume of said sintered body, said Y₄ Al₂ O₉phase ranging from a trace amount to less than about 6.0% by volume ofsaid sintered body, said body having a porosity of less than about 10%by volume of said body and a thermal conductivity greater than 1.00W/cm.K at 25° C.
 33. A polycrystalline body having a phase compositioncomprised of AlN and Y₄ Al₂ O₉ wherein the amount of said Y₄ Al₂ O₉phase ranges from about 0.8% by volume to less than about 2.1% by volumeof the total volume of said body, said body having a porosity of lessthan about 10% by volume of said body and a thermal conductivity greaterthan 1.00 W/cm.K at 25° C.
 34. A polycrystalline body having a phasecomposition comprised of AlN and Y₄ Al₂ O₉ wherein the amount of said Y₄Al₂ O₉ phase ranges from about 2.1% by volume to less than about 6.0% byvolume of the total volume of said body, said body having a porosity ofless than about 10% by volume of said body and a thermal conductivitygreater than 1.00 W/cm.K at 25° C.
 35. A polycrystalline body having aphase composition comprised of AlN, YAlO₃ and Y₄ Al₂ O₉ wherein thetotal amount of YAlO₃ and Y₄ Al₂ O₉ phases ranges from greater thanabout 0.8% by volume to less than about 2.1% by volume of the totalvolume of said body, said YAlO₃ phase ranging from a trace amount toless than about 1.7% by volume of the sintered body, said Y₄ Al₂ O₉phase ranging from a trace amount to less than about 2.1% by volume ofthe sintered body, said body having a porosity of less than about 10% byvolume of said body and a thermal conductivity greater than 1.00 W/cm.Kat 25° C.
 36. A polycrystalline body having a phase compositioncomprised of AlN, YAlO₃ and Y₄ Al₂ O₉ wherein the total amount of YAlO₃and Y₄ Al₂ O₉ phase ranges from greater than about 1.7% by volume toless than about 6.0% by volume of the total volume of said body, saidYAlO₃ phase ranging from a trace amount to less than about 4.2% byvolume of the sintered body, said Y₄ Al₂ O₉ phase ranging from a traceamount to less than about 6.0% by volume of the sintered body, said bodyhaving a porosity of less than about 10% by volume of said body and athermal conductivity greater than 1.00 W/cm.K at 25° C.
 37. Thepolycrystalline body according to claim 36 wherein said body containscarbon in an amount of less than 0.04% by weight of said body and has aporosity of less than about 1% by volume of said body and a thermalconductivity greater than 1.57 W/cm.K at 25° C. Two of the compacts wereplaced side by side on a molybedenum plate.